diff --git a/src/cpl_query/extension/list.py b/src/cpl_query/extension/list.py
index b216b18f..7496d43c 100644
--- a/src/cpl_query/extension/list.py
+++ b/src/cpl_query/extension/list.py
@@ -10,3 +10,9 @@ class List(Iterable):
 
         if values is not None:
             self.extend(values)
+
+    def append(self, __object: object) -> None:
+        if self._type is not None and type(__object) != self._type and not isinstance(type(__object), self._type):
+            raise Exception(f'Unexpected type: {type(__object)}')
+
+        super().append(__object)
diff --git a/src/cpl_query/tests/iterable_test.py b/src/cpl_query/tests/iterable_test.py
new file mode 100644
index 00000000..9b12b28e
--- /dev/null
+++ b/src/cpl_query/tests/iterable_test.py
@@ -0,0 +1,28 @@
+import unittest
+
+from cpl_query.extension.list import List
+
+
+class IterableTest(unittest.TestCase):
+
+    def setUp(self) -> None:
+        self._list = List(int)
+
+    def _clear(self):
+        self._list.clear()
+        self.assertEqual(self._list, [])
+
+    def test_append(self):
+        self._list.append(1)
+        self._list.append(2)
+        self._list.append(3)
+
+        self.assertEqual(self._list, [1, 2, 3])
+        self._clear()
+
+    def test_append_wrong_type(self):
+        self._list.append(1)
+        self._list.append(2)
+
+        self.assertRaises(Exception, lambda v: self._list.append(v), '3')
+        self._clear()
diff --git a/src/cpl_query/tests/query_test.py b/src/cpl_query/tests/query_test.py
index 2d0e8784..9135c434 100644
--- a/src/cpl_query/tests/query_test.py
+++ b/src/cpl_query/tests/query_test.py
@@ -143,5 +143,4 @@ class QueryTest(unittest.TestCase):
                 results.append(user)
 
         res = self._tests.where(lambda u: u.address.nr == 5)
-        # res = self._tests.where('User.address.nr == 5')
         self.assertEqual(len(results), len(res))
diff --git a/src/cpl_query/tests/tester.py b/src/cpl_query/tests/tester.py
index 98b10ebb..084071c3 100644
--- a/src/cpl_query/tests/tester.py
+++ b/src/cpl_query/tests/tester.py
@@ -1,5 +1,6 @@
 import unittest
 
+from cpl_query.tests.iterable_test import IterableTest
 from cpl_query.tests.query_test import QueryTest
 
 
@@ -11,6 +12,7 @@ class Tester:
     def create(self):
         loader = unittest.TestLoader()
         self._suite.addTests(loader.loadTestsFromTestCase(QueryTest))
+        self._suite.addTests(loader.loadTestsFromTestCase(IterableTest))
 
     def start(self):
         runner = unittest.TextTestRunner()