diff --git a/samples/linkedlist.js b/samples/linkedlist.js
index f3aa7f2..8693dcd 100644
--- a/samples/linkedlist.js
+++ b/samples/linkedlist.js
@@ -16,7 +16,7 @@ function append(lst: list, item: number) {
 
 function getOdd(lst: list): number[] {
     let result: number[]
-    let n = lst.head
+    let n: node = lst.head
     while (n != null) {
         if (n.data % 2 != 0) {
             let i = n.data
diff --git a/tests/HydraScript.IntegrationTests/ErrorPrograms/NullAssignmentWhenUndefinedTests.cs b/tests/HydraScript.IntegrationTests/ErrorPrograms/NullAssignmentWhenUndefinedTests.cs
new file mode 100644
index 0000000..5f02cb4
--- /dev/null
+++ b/tests/HydraScript.IntegrationTests/ErrorPrograms/NullAssignmentWhenUndefinedTests.cs
@@ -0,0 +1,32 @@
+using System.CommandLine.Parsing;
+
+namespace HydraScript.IntegrationTests.ErrorPrograms;
+
+public class NullAssignmentWhenUndefinedTests(TestHostFixture fixture) : IClassFixture<TestHostFixture>
+{
+    [Theory, MemberData(nameof(NullAssignmentScripts))]
+    public void NullAssignment_UndefinedDestinationOrReturnType_HydraScriptError(string script)
+    {
+        var runner = fixture.GetRunner(
+            configureTestServices: services =>
+                services.SetupInMemoryScript(script));
+        var code = runner.Invoke(fixture.InMemoryScript);
+        code.Should().Be(ExitCodes.HydraScriptError);
+        fixture.LogMessages.Should()
+            .Contain(x => x.Contains("Cannot assign 'null' when type is undefined"));
+    }
+
+    public static TheoryData<string> NullAssignmentScripts
+    {
+        get
+        {
+            const string lexicalDeclaration = "let x = null";
+            const string objectLiteralProperty = "let y = {prop: null;}";
+            const string functionReturn = "function f() { return null }";
+            return new TheoryData<string>([
+                lexicalDeclaration,
+                objectLiteralProperty,
+                functionReturn]);
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/HydraScript.IntegrationTests/ErrorPrograms/VariableInitializationTests.cs b/tests/HydraScript.IntegrationTests/ErrorPrograms/VariableInitializationTests.cs
index bde303d..ab00ac5 100644
--- a/tests/HydraScript.IntegrationTests/ErrorPrograms/VariableInitializationTests.cs
+++ b/tests/HydraScript.IntegrationTests/ErrorPrograms/VariableInitializationTests.cs
@@ -5,7 +5,7 @@ namespace HydraScript.IntegrationTests.ErrorPrograms;
 public class VariableInitializationTests(TestHostFixture fixture) : IClassFixture<TestHostFixture>
 {
     [Theory, MemberData(nameof(VariableInitializationScripts))]
-    public void VariableWithoutTypeDeclared_AccessedBeforeInitialization_ExitCodeHydraScriptError(string script)
+    public void VariableWithoutTypeDeclared_AccessedBeforeInitialization_HydraScriptError(string script)
     {
         var runner = fixture.GetRunner(
             configureTestServices: services =>
@@ -36,7 +36,9 @@ function f() {
                     return 5
                 }
                 """;
-            return new TheoryData<string>([variableWithoutTypeDeclared, typedVariableDeclared]);
+            return new TheoryData<string>([
+                variableWithoutTypeDeclared,
+                typedVariableDeclared]);
         }
     }
 }
\ No newline at end of file
diff --git a/tests/HydraScript.IntegrationTests/ErrorPrograms/VoidAssignmentTests.cs b/tests/HydraScript.IntegrationTests/ErrorPrograms/VoidAssignmentTests.cs
index 43af6a1..9ef8ba9 100644
--- a/tests/HydraScript.IntegrationTests/ErrorPrograms/VoidAssignmentTests.cs
+++ b/tests/HydraScript.IntegrationTests/ErrorPrograms/VoidAssignmentTests.cs
@@ -5,7 +5,7 @@ namespace HydraScript.IntegrationTests.ErrorPrograms;
 public class VoidAssignmentTests(TestHostFixture fixture) : IClassFixture<TestHostFixture>
 {
     [Fact]
-    public void FunctionDeclared_VoidReturnAssigned_ExitCodeHydraScriptError()
+    public void FunctionDeclared_VoidReturnAssigned_HydraScriptError()
     {
         const string script =
             """