diff --git a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IndexOfTest.php b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IndexOfTest.php
index dcc3e1c48..1403ab698 100644
--- a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IndexOfTest.php
+++ b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IndexOfTest.php
@@ -42,6 +42,18 @@ public function test_index_of() : void
         );
     }
 
+    public function test_needle_null_index_of_returns_false() : void
+    {
+        self::assertFalse(
+            ref('str')->indexOf(ref('needle'))->eval(
+                row(
+                    str_entry('str', 'x'),
+                    str_entry('needle', null)
+                )
+            )
+        );
+    }
+
     public function test_returns_method_returns_string_int() : void
     {
         $indexOf = new IndexOf('Abba', 'b', offset: 3);
@@ -51,4 +63,15 @@ public function test_returns_method_returns_string_int() : void
 
         self::assertTrue($returnType->isEqual(type_int()));
     }
+
+    public function test_string_null_index_of_returns_false() : void
+    {
+        self::assertFalse(
+            ref('str')->indexOf('x')->eval(
+                row(
+                    str_entry('str', null),
+                )
+            )
+        );
+    }
 }
diff --git a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IsUtf8Test.php b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IsUtf8Test.php
index 66d2889b1..dca635d67 100644
--- a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IsUtf8Test.php
+++ b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/IsUtf8Test.php
@@ -12,6 +12,17 @@
 
 final class IsUtf8Test extends FlowTestCase
 {
+    public function test_is_utf8_returns_null() : void
+    {
+        self::assertNull(
+            ref('str')->isUtf8()->eval(
+                row(
+                    str_entry('str', null),
+                )
+            )
+        );
+    }
+
     public function test_is_utf_8() : void
     {
         self::assertTrue(
diff --git a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringAfterTest.php b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringAfterTest.php
index f556769fc..569f76125 100644
--- a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringAfterTest.php
+++ b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringAfterTest.php
@@ -6,7 +6,7 @@
 
 use function Flow\ETL\DSL\row;
 use function Flow\ETL\DSL\{ref, str_entry, type_string};
-use Flow\ETL\Function\StringTitle;
+use Flow\ETL\Function\{StringAfter};
 use Flow\ETL\PHP\Type\Type;
 use Flow\ETL\Tests\FlowTestCase;
 
@@ -14,8 +14,8 @@ final class StringAfterTest extends FlowTestCase
 {
     public function test_returns_method_returns_string_type() : void
     {
-        $stringTitleFunction = new StringTitle('str');
-        $returnType = $stringTitleFunction->returns();
+        $stringAfterFunction = new StringAfter('test', 'e');
+        $returnType = $stringAfterFunction->returns();
 
         self::assertInstanceOf(Type::class, $returnType);
 
@@ -57,4 +57,15 @@ public function test_string_after_including_needle() : void
             )
         );
     }
+
+    public function test_string_after_returns_null() : void
+    {
+        self::assertNull(
+            ref('str')->stringAfter('x')->eval(
+                row(
+                    str_entry('str', null),
+                )
+            )
+        );
+    }
 }
diff --git a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringFoldTest.php b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringFoldTest.php
index 5721d611a..70577fbe0 100644
--- a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringFoldTest.php
+++ b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringFoldTest.php
@@ -5,11 +5,23 @@
 namespace Flow\ETL\Tests\Unit\Function;
 
 use function Flow\ETL\DSL\row;
-use function Flow\ETL\DSL\{ref, str_entry};
+use function Flow\ETL\DSL\{ref, str_entry, type_string};
+use Flow\ETL\Function\StringFold;
+use Flow\ETL\PHP\Type\Type;
 use Flow\ETL\Tests\FlowTestCase;
 
 final class StringFoldTest extends FlowTestCase
 {
+    public function test_returns_method_returns_string_type() : void
+    {
+        $stringFoldedFunction = new StringFold('str');
+        $returnType = $stringFoldedFunction->returns();
+
+        self::assertInstanceOf(Type::class, $returnType);
+
+        self::assertTrue($returnType->isEqual(type_string()));
+    }
+
     public function test_string_folded() : void
     {
         self::assertSame(
@@ -19,4 +31,15 @@ public function test_string_folded() : void
             )
         );
     }
+
+    public function test_string_folded_returns_null() : void
+    {
+        self::assertNull(
+            ref('str')->stringFold()->eval(
+                row(
+                    str_entry('str', null),
+                )
+            )
+        );
+    }
 }
diff --git a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringStyleTest.php b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringStyleTest.php
index 5448c5e3e..e48405f21 100644
--- a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringStyleTest.php
+++ b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringStyleTest.php
@@ -56,6 +56,17 @@ public function test_string_style_lower() : void
         );
     }
 
+    public function test_string_style_returns_null() : void
+    {
+        self::assertNull(
+            ref('str')->stringStyle(StringStyles::LOWER)->eval(
+                row(
+                    str_entry('str', null),
+                )
+            )
+        );
+    }
+
     public function test_string_style_snake() : void
     {
         self::assertSame(
diff --git a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringTitleTest.php b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringTitleTest.php
index 766bc4102..92a6f68e5 100644
--- a/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringTitleTest.php
+++ b/src/core/etl/tests/Flow/ETL/Tests/Unit/Function/StringTitleTest.php
@@ -41,4 +41,15 @@ public function test_string_title_allwords() : void
             )
         );
     }
+
+    public function test_string_title_returns_null() : void
+    {
+        self::assertNull(
+            ref('str')->stringTitle()->eval(
+                row(
+                    str_entry('str', null),
+                )
+            )
+        );
+    }
 }