diff --git a/CI/azure-pipelines.yml b/CI/azure-pipelines.yml
index ad878878c..bd7f3304b 100644
--- a/CI/azure-pipelines.yml
+++ b/CI/azure-pipelines.yml
@@ -71,7 +71,7 @@ jobs:
 
     - task: PublishPipelineArtifact@1
       inputs:
-        path: xFunc.Maths/bin/Release/netstandard2.1
+        path: xFunc.Maths/bin/Release/net6.0
         artifact: xFunc.Maths
 
     - task: PublishPipelineArtifact@1
diff --git a/xFunc.DotnetTool/xFunc.DotnetTool.csproj b/xFunc.DotnetTool/xFunc.DotnetTool.csproj
index 309bfead9..4365e6ee5 100644
--- a/xFunc.DotnetTool/xFunc.DotnetTool.csproj
+++ b/xFunc.DotnetTool/xFunc.DotnetTool.csproj
@@ -7,7 +7,7 @@
     <PackAsTool>true</PackAsTool>
     <ToolCommandName>xFunc</ToolCommandName>    
     <PackageId>xFunc.DotnetTool</PackageId>
-    <Version>4.1.0</Version>
+    <Version>4.2.0</Version>
     <Product>xFunc.Maths</Product>
     <Authors>Dmytro Kyshchenko</Authors>
     <Company />
@@ -15,8 +15,8 @@
     <PackageProjectUrl>https://github.com/sys27/xFunc</PackageProjectUrl>
     <PackageIcon>xFunc.png</PackageIcon>
     <PackageTags>Math Expression Expressions Parser MathParser Differentiation Simplification</PackageTags>
-    <AssemblyVersion>4.1.0</AssemblyVersion>
-    <FileVersion>4.1.0</FileVersion>
+    <AssemblyVersion>4.2.0</AssemblyVersion>
+    <FileVersion>4.2.0</FileVersion>
     <RepositoryUrl>https://github.com/sys27/xFunc</RepositoryUrl>
     <RepositoryType>GitHub</RepositoryType>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
diff --git a/xFunc.Maths/Expressions/Units/AngleUnits/AngleUnit.cs b/xFunc.Maths/Expressions/Units/AngleUnits/AngleUnit.cs
index d160d1e24..96868d206 100644
--- a/xFunc.Maths/Expressions/Units/AngleUnits/AngleUnit.cs
+++ b/xFunc.Maths/Expressions/Units/AngleUnits/AngleUnit.cs
@@ -50,12 +50,28 @@ private AngleUnit(double factor, params string[] unitNames)
         => !left.Equals(right);
 
     /// <inheritdoc />
-    public bool Equals(AngleUnit other)
-        => Factor.Equals(other.Factor) && UnitNames.SequenceEqual(other.UnitNames);
+    public bool Equals(AngleUnit? other)
+    {
+        if (ReferenceEquals(null, other))
+            return false;
+        if (ReferenceEquals(this, other))
+            return true;
+
+        return Factor.Equals(other.Factor) && UnitNames.Equals(other.UnitNames);
+    }
 
     /// <inheritdoc />
     public override bool Equals(object? obj)
-        => obj is AngleUnit other && Equals(other);
+    {
+        if (ReferenceEquals(null, obj))
+            return false;
+        if (ReferenceEquals(this, obj))
+            return true;
+        if (obj.GetType() != GetType())
+            return false;
+
+        return Equals((AngleUnit)obj);
+    }
 
     /// <inheritdoc />
     [ExcludeFromCodeCoverage]
@@ -116,7 +132,7 @@ static void AddUnits(IDictionary<string, AngleUnit> dictionary, AngleUnit unit)
     /// <param name="unit">When this method returns, the value associated with the specified name, if the unit is found; otherwise, the default value for the type of the value parameter. This parameter is passed uninitialized.</param>
     /// <returns><c>true</c> if area units contain an unit with the specified <paramref name="name"/>; otherwise, <c>false</c>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="name"/> is <c>null</c>.</exception>
-    public static bool FromName(string name, out AngleUnit unit)
+    public static bool FromName(string name, [NotNullWhen(true)] out AngleUnit? unit)
     {
         if (string.IsNullOrWhiteSpace(name))
             throw new ArgumentNullException(nameof(name));
diff --git a/xFunc.Maths/Expressions/Units/AreaUnits/AreaUnit.cs b/xFunc.Maths/Expressions/Units/AreaUnits/AreaUnit.cs
index 1ac583eae..1eb044ddd 100644
--- a/xFunc.Maths/Expressions/Units/AreaUnits/AreaUnit.cs
+++ b/xFunc.Maths/Expressions/Units/AreaUnits/AreaUnit.cs
@@ -85,12 +85,31 @@ private AreaUnit(double factor, string unitName)
         => !left.Equals(right);
 
     /// <inheritdoc />
-    public bool Equals(AreaUnit other)
-        => Factor.Equals(other.Factor) && UnitName == other.UnitName;
+    public bool Equals(AreaUnit? other)
+    {
+        if (ReferenceEquals(null, other))
+            return false;
+
+        if (ReferenceEquals(this, other))
+            return true;
+
+        return Factor.Equals(other.Factor) && UnitName == other.UnitName;
+    }
 
     /// <inheritdoc />
     public override bool Equals(object? obj)
-        => obj is AreaUnit other && Equals(other);
+    {
+        if (ReferenceEquals(null, obj))
+            return false;
+
+        if (ReferenceEquals(this, obj))
+            return true;
+
+        if (obj.GetType() != GetType())
+            return false;
+
+        return Equals((AreaUnit)obj);
+    }
 
     /// <inheritdoc />
     [ExcludeFromCodeCoverage]
@@ -173,7 +192,7 @@ private static IDictionary<string, AreaUnit> GetUnits()
     /// <param name="unit">When this method returns, the value associated with the specified name, if the unit is found; otherwise, the default value for the type of the value parameter. This parameter is passed uninitialized.</param>
     /// <returns><c>true</c> if area units contain an unit with the specified <paramref name="name"/>; otherwise, <c>false</c>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="name"/> is <c>null</c>.</exception>
-    public static bool FromName(string name, out AreaUnit unit)
+    public static bool FromName(string name, [NotNullWhen(true)] out AreaUnit? unit)
     {
         if (string.IsNullOrWhiteSpace(name))
             throw new ArgumentNullException(nameof(name));
diff --git a/xFunc.Maths/Expressions/Units/AreaUnits/AreaValue.cs b/xFunc.Maths/Expressions/Units/AreaUnits/AreaValue.cs
index 19aff6af0..39a861d07 100644
--- a/xFunc.Maths/Expressions/Units/AreaUnits/AreaValue.cs
+++ b/xFunc.Maths/Expressions/Units/AreaUnits/AreaValue.cs
@@ -204,7 +204,7 @@ public int CompareTo(AreaValue other)
     }
 
     /// <inheritdoc />
-    public int CompareTo(object obj)
+    public int CompareTo(object? obj)
         => obj switch
         {
             null => 1,
diff --git a/xFunc.Maths/Expressions/Units/LengthUnits/LengthValue.cs b/xFunc.Maths/Expressions/Units/LengthUnits/LengthValue.cs
index 5ea6c2180..fd1304451 100644
--- a/xFunc.Maths/Expressions/Units/LengthUnits/LengthValue.cs
+++ b/xFunc.Maths/Expressions/Units/LengthUnits/LengthValue.cs
@@ -316,7 +316,7 @@ public int CompareTo(LengthValue other)
     }
 
     /// <inheritdoc />
-    public int CompareTo(object obj)
+    public int CompareTo(object? obj)
         => obj switch
         {
             null => 1,
diff --git a/xFunc.Maths/Expressions/Units/MassUnits/MassValue.cs b/xFunc.Maths/Expressions/Units/MassUnits/MassValue.cs
index 947e35b12..1942e8b28 100644
--- a/xFunc.Maths/Expressions/Units/MassUnits/MassValue.cs
+++ b/xFunc.Maths/Expressions/Units/MassUnits/MassValue.cs
@@ -140,7 +140,7 @@ public int CompareTo(MassValue other)
     }
 
     /// <inheritdoc />
-    public int CompareTo(object obj)
+    public int CompareTo(object? obj)
         => obj switch
         {
             null => 1,
diff --git a/xFunc.Maths/Expressions/Units/PowerUnits/PowerValue.cs b/xFunc.Maths/Expressions/Units/PowerUnits/PowerValue.cs
index 7f6e9d8c6..4e7987eea 100644
--- a/xFunc.Maths/Expressions/Units/PowerUnits/PowerValue.cs
+++ b/xFunc.Maths/Expressions/Units/PowerUnits/PowerValue.cs
@@ -92,7 +92,7 @@ public int CompareTo(PowerValue other)
     }
 
     /// <inheritdoc />
-    public int CompareTo(object obj)
+    public int CompareTo(object? obj)
         => obj switch
         {
             null => 1,
diff --git a/xFunc.Maths/Expressions/Units/TimeUnits/TimeValue.cs b/xFunc.Maths/Expressions/Units/TimeUnits/TimeValue.cs
index 44dd4b1ba..b9eb65e72 100644
--- a/xFunc.Maths/Expressions/Units/TimeUnits/TimeValue.cs
+++ b/xFunc.Maths/Expressions/Units/TimeUnits/TimeValue.cs
@@ -188,7 +188,7 @@ public int CompareTo(TimeValue other)
     }
 
     /// <inheritdoc />
-    public int CompareTo(object obj)
+    public int CompareTo(object? obj)
         => obj switch
         {
             null => 1,
diff --git a/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeUnit.cs b/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeUnit.cs
index 27b0aed1f..0052a9306 100644
--- a/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeUnit.cs
+++ b/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeUnit.cs
@@ -70,12 +70,31 @@ private VolumeUnit(double factor, string unitName)
         => !left.Equals(right);
 
     /// <inheritdoc />
-    public bool Equals(VolumeUnit other)
-        => Factor.Equals(other.Factor) && UnitName == other.UnitName;
+    public bool Equals(VolumeUnit? other)
+    {
+        if (ReferenceEquals(null, other))
+            return false;
+
+        if (ReferenceEquals(this, other))
+            return true;
+
+        return Factor.Equals(other.Factor) && UnitName == other.UnitName;
+    }
 
     /// <inheritdoc />
     public override bool Equals(object? obj)
-        => obj is VolumeUnit other && Equals(other);
+    {
+        if (ReferenceEquals(null, obj))
+            return false;
+
+        if (ReferenceEquals(this, obj))
+            return true;
+
+        if (obj.GetType() != GetType())
+            return false;
+
+        return Equals((VolumeUnit)obj);
+    }
 
     /// <inheritdoc />
     [ExcludeFromCodeCoverage]
@@ -129,7 +148,7 @@ private static IDictionary<string, VolumeUnit> GetUnits()
     /// <param name="unit">When this method returns, the value associated with the specified name, if the unit is found; otherwise, the default value for the type of the value parameter. This parameter is passed uninitialized.</param>
     /// <returns><c>true</c> if volume units contain an unit with the specified <paramref name="name"/>; otherwise, <c>false</c>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="name"/> is <c>null</c>.</exception>
-    public static bool FromName(string name, out VolumeUnit unit)
+    public static bool FromName(string name, [NotNullWhen(true)] out VolumeUnit? unit)
     {
         if (string.IsNullOrWhiteSpace(name))
             throw new ArgumentNullException(nameof(name));
diff --git a/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeValue.cs b/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeValue.cs
index 0e752b2f0..a4dc825d2 100644
--- a/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeValue.cs
+++ b/xFunc.Maths/Expressions/Units/VolumeUnits/VolumeValue.cs
@@ -156,7 +156,7 @@ public int CompareTo(VolumeValue other)
     }
 
     /// <inheritdoc />
-    public int CompareTo(object obj)
+    public int CompareTo(object? obj)
         => obj switch
         {
             null => 1,
diff --git a/xFunc.Maths/Parser.ExpressionFactory.cs b/xFunc.Maths/Parser.ExpressionFactory.cs
index a1a4b3f41..66dd25056 100644
--- a/xFunc.Maths/Parser.ExpressionFactory.cs
+++ b/xFunc.Maths/Parser.ExpressionFactory.cs
@@ -15,7 +15,7 @@ public partial class Parser
 {
     private IExpression CreateFunction(in Token token, ImmutableArray<IExpression> arguments)
     {
-        Debug.Assert(token.IsId(), "Token should be Id.");
+        Debug.Assert(token.Is(Id), "Token should be Id.");
         Debug.Assert(!string.IsNullOrWhiteSpace(token.StringValue), "Id is empty.");
 
         return token.StringValue.ToLowerInvariant() switch
diff --git a/xFunc.Maths/Results/ExpressionResult.cs b/xFunc.Maths/Results/ExpressionResult.cs
index 7e2c249fb..8c7f44ebb 100644
--- a/xFunc.Maths/Results/ExpressionResult.cs
+++ b/xFunc.Maths/Results/ExpressionResult.cs
@@ -16,7 +16,7 @@ public ExpressionResult(IExpression exp)
         => Result = exp ?? throw new ArgumentNullException(nameof(exp));
 
     /// <inheritdoc />
-    public override string ToString() => Result.ToString();
+    public override string ToString() => Result.ToString()!;
 
     /// <inheritdoc cref="IResult.Result" />
     public IExpression Result { get; }
diff --git a/xFunc.Maths/Tokenization/Token.cs b/xFunc.Maths/Tokenization/Token.cs
index d074b2070..2766be38a 100644
--- a/xFunc.Maths/Tokenization/Token.cs
+++ b/xFunc.Maths/Tokenization/Token.cs
@@ -77,10 +77,10 @@ public static Token Create(TokenKind kind)
     [ExcludeFromCodeCoverage]
     public override string ToString()
     {
-        if (IsId() || Is(TokenKind.String))
+        if (Is(TokenKind.Id) || Is(TokenKind.String))
             return $"String: {StringValue}";
 
-        if (IsNumber())
+        if (Is(TokenKind.Number))
             return $"Number: {NumberValue}";
 
         return $"Kind: {Kind}";
@@ -161,24 +161,6 @@ public override string ToString()
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public bool IsNotEmpty() => !Is(TokenKind.Empty);
 
-    /// <summary>
-    /// Determines whether the current token is <c>Id</c> token.
-    /// </summary>
-    /// <returns>
-    /// <c>true</c> if the current token is <c>Id</c> token; otherwise, <c>false</c>.
-    /// </returns>
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public bool IsId() => Is(TokenKind.Id);
-
-    /// <summary>
-    /// Determines whether the current token is <c>Number</c> token.
-    /// </summary>
-    /// <returns>
-    /// <c>true</c> if the current token is <c>Number</c> token; otherwise, <c>false</c>.
-    /// </returns>
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public bool IsNumber() => Is(TokenKind.Number);
-
     /// <summary>
     /// Gets a token kind.
     /// </summary>
diff --git a/xFunc.Maths/xFunc.Maths.csproj b/xFunc.Maths/xFunc.Maths.csproj
index 4f58d3970..e9e074846 100644
--- a/xFunc.Maths/xFunc.Maths.csproj
+++ b/xFunc.Maths/xFunc.Maths.csproj
@@ -1,11 +1,11 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.1</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <LangVersion>latest</LangVersion>
     <RootNamespace>xFunc.Maths</RootNamespace>
     <PackageId>xFunc.Maths</PackageId>
-    <Version>4.1.0</Version>
+    <Version>4.2.0</Version>
     <Product>xFunc.Maths</Product>
     <Authors>Dmytro Kyshchenko</Authors>
     <Company />
@@ -15,8 +15,8 @@
     <PackageIcon>xFunc.png</PackageIcon>
     <PackageTags>Math Expression Expressions Parser MathParser Differentiation Simplification</PackageTags>
     <NeutralLanguage>en</NeutralLanguage>
-    <AssemblyVersion>4.1.0</AssemblyVersion>
-    <FileVersion>4.1.0</FileVersion>
+    <AssemblyVersion>4.2.0</AssemblyVersion>
+    <FileVersion>4.2.0</FileVersion>
     <RepositoryUrl>https://github.com/sys27/xFunc</RepositoryUrl>
     <RepositoryType>GitHub</RepositoryType>
     <PackageReleaseNotes>https://github.com/sys27/xFunc/wiki/ChangeLog
diff --git a/xFunc.Tests/Expressions/Matrices/CrossProductTests.cs b/xFunc.Tests/Expressions/Matrices/CrossProductTests.cs
index f3aef5d67..afd81b9ad 100644
--- a/xFunc.Tests/Expressions/Matrices/CrossProductTests.cs
+++ b/xFunc.Tests/Expressions/Matrices/CrossProductTests.cs
@@ -18,6 +18,28 @@ public void ExecuteTest()
         Assert.Equal(expected, result);
     }
 
+    [Fact]
+    public void ExecuteLeftLessThenThreeTest()
+    {
+        var exp = new CrossProduct(
+            new Vector(new[] { Number.One, Number.Two, }),
+            new Vector(new[] { new Number(4), new Number(5), new Number(6) })
+        );
+
+        Assert.Throws<ArgumentException>(() => exp.Execute());
+    }
+
+    [Fact]
+    public void ExecuteRightLessThenThreeTest()
+    {
+        var exp = new CrossProduct(
+            new Vector(new[] { Number.One, Number.Two, new Number(3) }),
+            new Vector(new[] { new Number(4), new Number(5), })
+        );
+
+        Assert.Throws<ArgumentException>(() => exp.Execute());
+    }
+
     [Fact]
     public void ExecuteTypeExceptionTest()
         => TestNotSupported(new CrossProduct(Number.One, Number.Two));
diff --git a/xFunc.Tests/Expressions/MulTest.cs b/xFunc.Tests/Expressions/MulTest.cs
index 0c0eb468e..d5bcb1a40 100644
--- a/xFunc.Tests/Expressions/MulTest.cs
+++ b/xFunc.Tests/Expressions/MulTest.cs
@@ -71,6 +71,22 @@ public void ExecuteDotProductTest()
         Assert.Equal(expected, exp.Execute());
     }
 
+    [Fact]
+    public void ExecuteDotProductErrorTest()
+    {
+        var vector1 = new Vector(new IExpression[]
+        {
+            Number.One, Number.Two,
+        });
+        var vector2 = new Vector(new IExpression[]
+        {
+            new Number(4), new Number(5), new Number(6)
+        });
+        var exp = new Mul(vector1, vector2);
+
+        Assert.Throws<ArgumentException>(() => exp.Execute());
+    }
+
     [Fact]
     public void ExecuteMulComplexByBool()
     {
diff --git a/xFunc.Tests/Expressions/Units/AngleUnits/AngleUnitTest.cs b/xFunc.Tests/Expressions/Units/AngleUnits/AngleUnitTest.cs
index b074bf4b4..266aa07e9 100644
--- a/xFunc.Tests/Expressions/Units/AngleUnits/AngleUnitTest.cs
+++ b/xFunc.Tests/Expressions/Units/AngleUnits/AngleUnitTest.cs
@@ -2,6 +2,14 @@ namespace xFunc.Tests.Expressions.Units.AngleUnits;
 
 public class AngleUnitTest
 {
+    [Fact]
+    public void EqualsNullTest()
+    {
+        var a = AngleUnit.Degree;
+
+        Assert.False(a.Equals(null));
+    }
+
     [Fact]
     public void EqualsTest()
     {
@@ -20,6 +28,14 @@ public void NotEqualsTest()
         Assert.False(a.Equals(b));
     }
 
+    [Fact]
+    public void ObjectEqualsNullTest()
+    {
+        var a = AngleUnit.Degree;
+
+        Assert.False(a.Equals(null as object));
+    }
+
     [Fact]
     public void ObjectEqualsTest()
     {
diff --git a/xFunc.Tests/Expressions/Units/AreaUnits/AreaUnitTest.cs b/xFunc.Tests/Expressions/Units/AreaUnits/AreaUnitTest.cs
index 1886e1942..8ea30272e 100644
--- a/xFunc.Tests/Expressions/Units/AreaUnits/AreaUnitTest.cs
+++ b/xFunc.Tests/Expressions/Units/AreaUnits/AreaUnitTest.cs
@@ -5,6 +5,14 @@ namespace xFunc.Tests.Expressions.Units.AreaUnits;
 
 public class AreaUnitTest
 {
+    [Fact]
+    public void EqualsNullTest()
+    {
+        var a = AreaUnit.Meter;
+
+        Assert.False(a.Equals(null));
+    }
+
     [Fact]
     public void EqualsTest()
     {
@@ -23,6 +31,14 @@ public void NotEqualsTest()
         Assert.False(a.Equals(b));
     }
 
+    [Fact]
+    public void ObjectEqualsNullTest()
+    {
+        var a = AreaUnit.Meter;
+
+        Assert.False(a.Equals(null as object));
+    }
+
     [Fact]
     public void ObjectEqualsTest()
     {
diff --git a/xFunc.Tests/Expressions/Units/VolumeUnits/VolumeUnitTest.cs b/xFunc.Tests/Expressions/Units/VolumeUnits/VolumeUnitTest.cs
index 5fc2f68a2..5bb4da71b 100644
--- a/xFunc.Tests/Expressions/Units/VolumeUnits/VolumeUnitTest.cs
+++ b/xFunc.Tests/Expressions/Units/VolumeUnits/VolumeUnitTest.cs
@@ -5,6 +5,14 @@ namespace xFunc.Tests.Expressions.Units.VolumeUnits;
 
 public class VolumeUnitTest
 {
+    [Fact]
+    public void EqualsNullTest()
+    {
+        var a = VolumeUnit.Meter;
+
+        Assert.False(a.Equals(null));
+    }
+
     [Fact]
     public void EqualsTest()
     {
@@ -23,6 +31,14 @@ public void NotEqualsTest()
         Assert.False(a.Equals(b));
     }
 
+    [Fact]
+    public void ObjectEqualsNullTest()
+    {
+        var a = VolumeUnit.Meter;
+
+        Assert.False(a.Equals(null as object));
+    }
+
     [Fact]
     public void ObjectEqualsTest()
     {
diff --git a/xFunc.sln b/xFunc.sln
index c5e13b984..a433c001a 100644
--- a/xFunc.sln
+++ b/xFunc.sln
@@ -19,6 +19,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 		.stylecop.json = .stylecop.json
 		Directory.Build.targets = Directory.Build.targets
 		Directory.Packages.props = Directory.Packages.props
+		version.json = version.json
 	EndProjectSection
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "CI", "CI", "{7D6A31E3-42CF-49E7-A0D2-657A460388BC}"