diff --git a/src/Model/JsonStringConvertible.php b/src/Model/JsonStringConvertible.php
index 5222474..8c9c712 100644
--- a/src/Model/JsonStringConvertible.php
+++ b/src/Model/JsonStringConvertible.php
@@ -14,9 +14,12 @@ public static function fromJSON(string $json): self
         return static::jsonDeserialize($object);
     }
 
-    public function toJSON(): string
+    public function toJSON(int $flags = 0): string
     {
-        return \Safe\json_encode(static::jsonSerialize());
+        return \Safe\json_encode(
+            static::jsonSerialize(),
+            $flags
+        );
     }
 
     abstract public static function jsonDeserialize(\stdClass $object): self;
diff --git a/src/Model/Value.php b/src/Model/Value.php
index d7eafeb..ad5204b 100644
--- a/src/Model/Value.php
+++ b/src/Model/Value.php
@@ -15,10 +15,10 @@ public function __construct(Document $document)
         $this->document = $document;
     }
 
-    public static function jsonDeserialize(\stdClass $value): self
+    public static function jsonDeserialize(\stdClass $object): self
     {
         return new self(
-            Document::jsonDeserialize($value->document)
+            Document::jsonDeserialize($object->document)
         );
     }
 
diff --git a/tests/DocumentTest.php b/tests/DocumentTest.php
index 6dbc211..4a707ae 100644
--- a/tests/DocumentTest.php
+++ b/tests/DocumentTest.php
@@ -8,7 +8,7 @@ public function testGetConcatenatedPlainTextContentOfInnerNodes(): void
     {
         $document = $this->loadDocumentFromFixture(__DIR__ . '/fixtures/document_with_text.json');
 
-        $this->assertEquals(
+        $this->assertSame(
             "I'd like to introduce you to a very important person!",
             $document->text
         );
diff --git a/tests/LeafTest.php b/tests/LeafTest.php
new file mode 100644
index 0000000..219de5f
--- /dev/null
+++ b/tests/LeafTest.php
@@ -0,0 +1,15 @@
+<?php declare(strict_types=1);
+
+namespace Prezly\Slate\Tests;
+
+use Prezly\Slate\Model\Leaf;
+
+final class LeafTest extends TestCase
+{
+    public function testToJSON(): void
+    {
+        $leaf = new Leaf('foo 😎');
+        $this->assertSame('{"object":"leaf","text":"foo 😎","marks":[]}', $leaf->toJSON(JSON_UNESCAPED_UNICODE));
+        $this->assertSame('{"object":"leaf","text":"foo \ud83d\ude0e","marks":[]}', $leaf->toJSON());
+    }
+}
diff --git a/tests/UnserializerTest.php b/tests/UnserializerTest.php
index 61a7c61..b5744a1 100644
--- a/tests/UnserializerTest.php
+++ b/tests/UnserializerTest.php
@@ -67,9 +67,9 @@ public function testAddChildrenToDocument(): void
             $this->assertInstanceOf(Block::class, $node);
         }
 
-        $this->assertEquals('paragraph', $nodes[0]->type);
-        $this->assertEquals('blockquote', $nodes[1]->type);
-        $this->assertEquals('list', $nodes[2]->type);
+        $this->assertSame('paragraph', $nodes[0]->type);
+        $this->assertSame('blockquote', $nodes[1]->type);
+        $this->assertSame('list', $nodes[2]->type);
     }
 
     public function testNestChildren(): void
@@ -109,7 +109,7 @@ public function testLoadDocumentWithLeaves(): void
 
         foreach ($text->leaves as $i => $leaf) {
             $this->assertInstanceOf(Leaf::class, $leaf);
-            $this->assertEquals($expected_texts[$i], $leaf->text);
+            $this->assertSame($expected_texts[$i], $leaf->text);
         }
     }
 
@@ -122,8 +122,8 @@ public function testSetNodeData(): void
             $block->data
         );
 
-        /** @var Inline $inline */
         $inline = $block->nodes[0];
+        assert($inline instanceof Inline);
         $this->assertEquals(
             (object) ['name' => 'John Doe', 'id' => 1234],
             $inline->data
diff --git a/tests/ValueTest.php b/tests/ValueTest.php
index 5264436..dfd4178 100644
--- a/tests/ValueTest.php
+++ b/tests/ValueTest.php
@@ -5,7 +5,7 @@
 final class ValueTest extends TestCase
 {
     /** @dataProvider fixtures */
-    public function testSerializeToJson(string $filePath): void
+    public function testSerializeToJSON(string $filePath): void
     {
         $json = $this->loadFixture($filePath);
         $value = $this->loadContentFromFixture($filePath);
@@ -18,7 +18,7 @@ public function testSerializeToJson(string $filePath): void
             )
         );
 
-        $this->assertEquals(trim($json), \Safe\json_encode($value, JSON_PRETTY_PRINT));
+        $this->assertSame(trim($json), \Safe\json_encode($value, JSON_PRETTY_PRINT));
     }
 
     /** @return iterable<array{string}> */