diff --git a/type.go b/type.go
index f4d03f99a..2e6936cd9 100644
--- a/type.go
+++ b/type.go
@@ -88,6 +88,15 @@ func (n binaryNode) Type(table typesTable) (Type, error) {
 		}
 		return nil, fmt.Errorf(`invalid operation: %v (mismatched types %v and %v)`, n, ltype, rtype)
 
+	case "in", "not in":
+		if (isStringType(ltype) || isInterfaceType(ltype)) && (isStructType(rtype) || isInterfaceType(rtype)) {
+			return boolType, nil
+		}
+		if isArrayType(rtype) || isMapType(rtype) || isInterfaceType(rtype) {
+			return boolType, nil
+		}
+		return nil, fmt.Errorf(`invalid operation: %v (mismatched types %v and %v)`, n, ltype, rtype)
+
 	case "<", ">", ">=", "<=":
 		if (isNumberType(ltype) || isInterfaceType(ltype)) && (isNumberType(rtype) || isInterfaceType(rtype)) {
 			return boolType, nil
@@ -325,6 +334,39 @@ func isStringType(t Type) bool {
 	return false
 }
 
+func isArrayType(t Type) bool {
+	t = dereference(t)
+	if t != nil {
+		switch t.Kind() {
+		case reflect.Slice, reflect.Array:
+			return true
+		}
+	}
+	return false
+}
+
+func isMapType(t Type) bool {
+	t = dereference(t)
+	if t != nil {
+		switch t.Kind() {
+		case reflect.Map:
+			return true
+		}
+	}
+	return false
+}
+
+func isStructType(t Type) bool {
+	t = dereference(t)
+	if t != nil {
+		switch t.Kind() {
+		case reflect.Struct:
+			return true
+		}
+	}
+	return false
+}
+
 func fieldType(ntype Type, name string) (Type, bool) {
 	ntype = dereference(ntype)
 	if ntype != nil {
diff --git a/type_test.go b/type_test.go
index e902bdfcc..46eb0d853 100644
--- a/type_test.go
+++ b/type_test.go
@@ -52,6 +52,10 @@ var typeTests = []typeTest{
 	"nil == nil",
 	"nil == IntPtr",
 	"Foo2p.Bar.Baz",
+	"Str in Foo",
+	"Str in Arr",
+	"nil in Arr",
+	"Str not in Foo2p",
 	"Int | Num",
 	"Int ^ Num",
 	"Int & Num",
@@ -268,6 +272,14 @@ var typeErrorTests = []typeErrorTest{
 		"NilFn() and OkFn()",
 		"invalid operation: NilFn() and OkFn() (mismatched types <nil> and bool)",
 	},
+	{
+		"'str' in Str",
+		`invalid operation: "str" in Str (mismatched types string and string)`,
+	},
+	{
+		"1 in Foo",
+		"invalid operation: 1 in Foo (mismatched types float64 and *expr_test.foo)",
+	},
 }
 
 type abc interface {