diff --git a/ets2panda/checker/ETSAnalyzer.cpp b/ets2panda/checker/ETSAnalyzer.cpp
index 2c0fd6a3fc8343d00a78b72c70677cc2e13d5cbd..86485d23f652513ede9b8809b36e8e6d5cb607a0 100644
--- a/ets2panda/checker/ETSAnalyzer.cpp
+++ b/ets2panda/checker/ETSAnalyzer.cpp
@@ -2980,7 +2980,7 @@ checker::Type *ETSAnalyzer::Check(ir::ForOfStatement *const st) const
         return checker->GlobalTypeError();
     }
 
-    checker::Type *elemType = nullptr;
+    checker::Type *elemType = checker->GlobalTypeError();
 
     if (exprType->IsETSStringType()) {
         elemType = checker->GetGlobalTypesHolder()->GlobalCharType();
@@ -2990,7 +2990,7 @@ checker::Type *ETSAnalyzer::Check(ir::ForOfStatement *const st) const
         elemType = st->CheckIteratorMethod(checker);
     }
 
-    if (elemType == nullptr) {
+    if (elemType == checker->GlobalTypeError()) {
         checker->LogError(diagnostic::FOROF_SOURCE_NONITERABLE, {}, st->Right()->Start());
         return checker->GlobalTypeError();
     }
diff --git a/ets2panda/checker/types/ets/etsUnionType.cpp b/ets2panda/checker/types/ets/etsUnionType.cpp
index 2bbdf54442f4f6e4e746f7b52b6a3795a33aedc4..7893a517a96567f583f9e474efa25dbfa71d7f6d 100644
--- a/ets2panda/checker/types/ets/etsUnionType.cpp
+++ b/ets2panda/checker/types/ets/etsUnionType.cpp
@@ -265,6 +265,7 @@ void ETSUnionType::LinearizeAndEraseIdentical(TypeRelation *relation, ArenaVecto
     size_t const initialSz = types.size();
     for (size_t i = 0; i < initialSz; ++i) {
         auto ct = types[i];
+        ES2PANDA_ASSERT(ct != nullptr);
         if (ct->IsETSUnionType()) {
             auto const &otherTypes = ct->AsETSUnionType()->ConstituentTypes();
             types.insert(types.end(), otherTypes.begin(), otherTypes.end());
diff --git a/ets2panda/ir/statements/forOfStatement.cpp b/ets2panda/ir/statements/forOfStatement.cpp
index 31198b09a9755209b6fb1ade0c2322948c9db476..d6eee195c8143ddda3e66a62bceb22336dc81d13 100644
--- a/ets2panda/ir/statements/forOfStatement.cpp
+++ b/ets2panda/ir/statements/forOfStatement.cpp
@@ -35,7 +35,7 @@ checker::Type *ForOfStatement::CreateUnionIteratorTypes(checker::ETSChecker *che
                                                                              checker->GetGlobalTypesHolder()));
             types.back()->RemoveTypeFlag(checker::TypeFlag::CONSTANT);
         } else {
-            return nullptr;
+            return checker->GlobalTypeError();
         }
     }
 
@@ -158,7 +158,7 @@ checker::Type *ForOfStatement::CheckIteratorMethodForObject(checker::ETSChecker
     auto *const method = sourceType->GetProperty(compiler::Signatures::ITERATOR_METHOD, searchFlag);
     if (method == nullptr || !method->HasFlag(varbinder::VariableFlags::METHOD)) {
         checker->LogError(diagnostic::MISSING_ITERATOR_METHOD, {}, position);
-        return nullptr;
+        return checker->GlobalTypeError();
     }
 
     ArenaVector<Expression *> arguments {checker->Allocator()->Adapter()};
@@ -167,27 +167,27 @@ checker::Type *ForOfStatement::CheckIteratorMethodForObject(checker::ETSChecker
                                                                 checker::TypeRelationFlag::NO_THROW);
     if (signature == nullptr) {
         checker->LogError(diagnostic::MISSING_ITERATOR_METHOD_WITH_SIG, {}, position);
-        return nullptr;
+        return checker->GlobalTypeError();
     }
     checker->ValidateSignatureAccessibility(sourceType, signature, position, {{diagnostic::INVISIBLE_ITERATOR, {}}});
 
     ES2PANDA_ASSERT(signature->Function() != nullptr);
 
     if (!CheckReturnTypeOfIteratorMethod(checker, sourceType, signature, position)) {
-        return nullptr;
+        return checker->GlobalTypeError();
     }
 
     if (checker->IsClassStaticMethod(sourceType, signature)) {
         checker->LogError(diagnostic::PROP_IS_STATIC, {compiler::Signatures::ITERATOR_METHOD, sourceType->Name()},
                           position);
-        return nullptr;
+        return checker->GlobalTypeError();
     }
 
     auto *const nextMethod =
         signature->ReturnType()->AsETSObjectType()->GetProperty(ITERATOR_INTERFACE_METHOD, searchFlag);
     if (nextMethod == nullptr || !nextMethod->HasFlag(varbinder::VariableFlags::METHOD)) {
         checker->LogError(diagnostic::ITERATOR_MISSING_NEXT, {}, position);
-        return nullptr;
+        return checker->GlobalTypeError();
     }
 
     auto &nextSignatures = checker->GetTypeOfVariable(nextMethod)->AsETSFunctionType()->CallSignatures();
@@ -200,7 +200,7 @@ checker::Type *ForOfStatement::CheckIteratorMethodForObject(checker::ETSChecker
         }
     }
 
-    return nullptr;
+    return checker->GlobalTypeError();
 }
 
 bool ForOfStatement::CheckReturnTypeOfIteratorMethod(checker::ETSChecker *checker, checker::ETSObjectType *sourceType,
@@ -251,6 +251,6 @@ checker::Type *ForOfStatement::CheckIteratorMethod(checker::ETSChecker *const ch
         }
     }
 
-    return nullptr;
+    return checker->GlobalTypeError();
 }
 }  // namespace ark::es2panda::ir
diff --git a/ets2panda/test/ast/compiler/ets/forof_iterator_doesnt_return_iteartor.ets b/ets2panda/test/ast/compiler/ets/forof_iterator_doesnt_return_iteartor.ets
new file mode 100644
index 0000000000000000000000000000000000000000..e340e0579b0de0ac364512cdacfe3c67a47909db
--- /dev/null
+++ b/ets2panda/test/ast/compiler/ets/forof_iterator_doesnt_return_iteartor.ets
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2025 Huawei Device Co., Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class A {
+    data: FixedArray<int> = [1,2,3];
+    $_iterator() : CIterator | undefined {
+      return new CIterator(this);
+    }
+}
+
+class CIterator implements Iterator<int> {
+    index = 0;
+    base: A;
+    constructor (base: A) {
+        this.base = base;
+    }
+    next(): IteratorResult<int> {
+        if (this.index >= this.base.data.length) {
+            return {
+                done: true,
+                value: undefined
+            }
+        }
+        return {
+          done: this.index >= this.base.data.length,
+          value: this.base.data[this.index++]
+        }
+    }
+}
+
+function main(): void {
+    let res = 0;
+    let a = new A();
+    for (let x of a) res += x;
+    arktest.assertEQ(res, 6);
+}
+
+
+/* @@? 18:15 Error TypeError: The return type of '$_iterator' must be a type that implements Iterator interface.*/
+/* @@? 46:19 Error TypeError: Iterator method must return an object which implements Iterator<T> */
+/* @@? 46:19 Error TypeError: 'For-of' statement source expression is not of iterable type. */
+
diff --git a/ets2panda/test/ast/compiler/ets/forof_iterator_missing_next.ets b/ets2panda/test/ast/compiler/ets/forof_iterator_missing_next.ets
new file mode 100644
index 0000000000000000000000000000000000000000..f513a77c6e85deebbe8b98aa645ed8d95c84e15f
--- /dev/null
+++ b/ets2panda/test/ast/compiler/ets/forof_iterator_missing_next.ets
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2025 Huawei Device Co., Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class A {
+    data: FixedArray<int> = [1,2,3];
+    $_iterator() : CIterator | undefined {
+      return new CIterator(this);
+    }
+}
+
+class CIterator  {
+    index = 0;
+    base: A;
+    constructor (base: A) {
+        this.base = base;
+    }
+}
+
+function main(): void {
+    let res = 0;
+    let a = new A();
+    for (let x of a) res += x;
+    arktest.assertEQ(res, 6);
+}
+
+/* @@? 18:15 Error TypeError: The return type of '$_iterator' must be a type that implements Iterator interface.*/
+/* @@? 34:19 Error TypeError: Iterator method must return an object which implements Iterator<T> */
+/* @@? 34:19 Error TypeError: 'For-of' statement source expression is not of iterable type. */
\ No newline at end of file
diff --git a/ets2panda/test/ast/compiler/ets/forof_missing_iterator_method.ets b/ets2panda/test/ast/compiler/ets/forof_missing_iterator_method.ets
new file mode 100644
index 0000000000000000000000000000000000000000..8952030938ed9e0299fe4d66d42505ead97d20d6
--- /dev/null
+++ b/ets2panda/test/ast/compiler/ets/forof_missing_iterator_method.ets
@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2025 Huawei Device Co., Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class C {
+  index = 0
+}
+
+function main(): int {
+  let c = new C()
+  let res = ''
+  for (let x of c) {
+      res += x
+  }
+  if (res != '6') return 1
+  return 0
+}
+
+/* @@? 23:17 Error TypeError: Object type doesn't have proper iterator method. */
+/* @@? 23:17 Error TypeError: 'For-of' statement source expression is not of iterable type. */
diff --git a/ets2panda/test/ast/compiler/ets/forof_missing_iterator_method_with_sig.ets b/ets2panda/test/ast/compiler/ets/forof_missing_iterator_method_with_sig.ets
new file mode 100644
index 0000000000000000000000000000000000000000..0adcb03bf1eb9a1fae580c5187ba574b6aa21732
--- /dev/null
+++ b/ets2panda/test/ast/compiler/ets/forof_missing_iterator_method_with_sig.ets
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2025 Huawei Device Co., Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class C {
+  $_iterator(param: int): string {
+    return "notAnIterator"; 
+  }
+}
+
+function main(): int {
+  let c = new C();
+  let res = "";
+
+  for (let x of c) { 
+    res += x;
+  }
+
+  if (res != "6") return 1;
+  return 0;
+}
+
+/* @@? 17:3 Error SyntaxError: The special predefined method '$_iterator' should not have parameters. */
+/* @@? 17:13 Error TypeError: The return type of '$_iterator' must be a type that implements Iterator interface. */
+/* @@? 26:17 Error TypeError: Cannot find iterator method with the required signature. */
+/* @@? 26:17 Error TypeError: 'For-of' statement source expression is not of iterable type. */
\ No newline at end of file
diff --git a/ets2panda/test/ast/compiler/ets/forof_prop_is_static.ets b/ets2panda/test/ast/compiler/ets/forof_prop_is_static.ets
new file mode 100644
index 0000000000000000000000000000000000000000..ff9986cb413d7253e4be1848410181fa7fe88526
--- /dev/null
+++ b/ets2panda/test/ast/compiler/ets/forof_prop_is_static.ets
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2025 Huawei Device Co., Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class C {
+  data: int[] = [1,2,3]
+  static $_iterator() {
+      return new CIterator(new C())
+  }
+}
+class CIterator implements Iterator<int> {
+  index = 0
+  base: C
+  constructor (base: C) {
+      this.base = base
+  }
+  next(): IteratorResult<int> {
+      return {
+          done: this.index >= this.base.data.length,
+          value: this.index >= this.base.data.length ? undefined : this.base.data[this.index++]
+      }
+  }
+}
+
+function main(): int {
+  let c = new C()
+  let res = ''
+  for (let x of c) {
+      res += x
+  }
+  if (res != '6') return 1
+  return 0
+}
+
+/* @@? 39:17 Error TypeError: '$_iterator' is a static property of 'C' */
+/* @@? 39:17 Error TypeError: 'For-of' statement source expression is not of iterable type. */
diff --git a/ets2panda/test/ast/compiler/ets/union_types_forof_1.ets b/ets2panda/test/ast/compiler/ets/union_types_forof_1.ets
new file mode 100644
index 0000000000000000000000000000000000000000..b13724b2d10945e3c15315d3c042c2b2c7793f73
--- /dev/null
+++ b/ets2panda/test/ast/compiler/ets/union_types_forof_1.ets
@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2025 Huawei Device Co., Ltd.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+ 
+function processValue(value: string | number): void {
+    if (typeof value === 'string') {
+    // Handle string case
+        for (const it of value) {
+            console.log(it);
+        }
+    } else {
+        // Handle number case
+        console.log(value.toString());
+    }
+}
+
+const testValues: (string | number)[] = ['hello', 42, 'world', 7];
+
+for (const item of testValues) {
+    processValue(item);
+}
+
+/* @@? 19:26 Error TypeError: Object type doesn't have proper iterator method. */