diff --git a/taiga/webhooks/serializers.py b/taiga/webhooks/serializers.py
index f72a92ba..01d0c025 100644
--- a/taiga/webhooks/serializers.py
+++ b/taiga/webhooks/serializers.py
@@ -141,6 +141,9 @@ def custom_attributes_queryset(self, project):
         raise NotImplementedError()
 
     def get_custom_attributes_values(self, obj):
+        if not obj:
+            return None
+
         def _use_name_instead_id_as_key_in_custom_attributes_values(custom_attributes, values):
             ret = {}
             for attr in custom_attributes:
diff --git a/tests/integration/test_webhook_serializers.py b/tests/integration/test_webhook_serializers.py
new file mode 100644
index 00000000..19f125e9
--- /dev/null
+++ b/tests/integration/test_webhook_serializers.py
@@ -0,0 +1,12 @@
+import taiga.webhooks.serializers
+
+
+class TestCustomAttributesValuesWebhookSerializerMixin:
+    webhook_serializer_mixin = (
+        taiga.webhooks.serializers.CustomAttributesValuesWebhookSerializerMixin
+    )
+
+    def test_get_custom_attributes_values(self):
+        assert (
+            self.webhook_serializer_mixin().get_custom_attributes_values(None) is None
+        )