diff --git a/django_mongodb_backend/functions.py b/django_mongodb_backend/functions.py
index 750a548f8..464dddd6f 100644
--- a/django_mongodb_backend/functions.py
+++ b/django_mongodb_backend/functions.py
@@ -102,6 +102,8 @@ def extract(self, compiler, connection):
 
 def func(self, compiler, connection):
     lhs_mql = process_lhs(self, compiler, connection)
+    if self.function is None:
+        raise NotSupportedError(f"{self} may need an as_mql() method.")
     operator = MONGO_OPERATORS.get(self.__class__, self.function.lower())
     return {f"${operator}": lhs_mql}
 
diff --git a/tests/db_functions_/__init__.py b/tests/db_functions_/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/db_functions_/tests.py b/tests/db_functions_/tests.py
new file mode 100644
index 000000000..23029d42e
--- /dev/null
+++ b/tests/db_functions_/tests.py
@@ -0,0 +1,10 @@
+from django.db import NotSupportedError
+from django.db.models import Func
+from django.test import SimpleTestCase
+
+
+class FuncTests(SimpleTestCase):
+    def test_no_as_mql(self):
+        msg = "Func() may need an as_mql() method."
+        with self.assertRaisesMessage(NotSupportedError, msg):
+            Func().as_mql(None, None)