diff --git a/openslides_backend/action/actions/user/user_mixins.py b/openslides_backend/action/actions/user/user_mixins.py
index c1b63a3e8..952fff3fe 100644
--- a/openslides_backend/action/actions/user/user_mixins.py
+++ b/openslides_backend/action/actions/user/user_mixins.py
@@ -123,7 +123,8 @@ def check_existence(what: str) -> None:
 
         check_existence("username")
         check_existence("saml_id")
-        check_existence("member_number")
+        if instance.get("member_number") is not None:
+            check_existence("member_number")
 
         self.check_meeting_and_users(
             instance, fqid_from_collection_and_id("user", instance["id"])
diff --git a/tests/system/action/user/test_create.py b/tests/system/action/user/test_create.py
index 7934ec72f..121ba0d08 100644
--- a/tests/system/action/user/test_create.py
+++ b/tests/system/action/user/test_create.py
@@ -361,6 +361,14 @@ def test_member_number_already_exists(self) -> None:
             == "A user with the member_number 14m4m3m832 already exists."
         )
 
+    def test_member_number_none(self) -> None:
+        response = self.request(
+            "user.create",
+            {"username": "user2", "member_number": None},
+        )
+        self.assert_status_code(response, 200)
+        self.assert_model_exists("user/2", {"member_number": None})
+
     def test_user_create_with_empty_vote_delegation_from_ids(self) -> None:
         self.set_models(
             {
diff --git a/tests/system/action/user/test_update.py b/tests/system/action/user/test_update.py
index f8f94d709..01c6282e6 100644
--- a/tests/system/action/user/test_update.py
+++ b/tests/system/action/user/test_update.py
@@ -549,6 +549,12 @@ def test_member_number_already_given(self) -> None:
             response.json["message"],
         )
 
+    def test_clear_member_number(self) -> None:
+        self.create_model("user/222", {"member_number": "klmnopqrst"})
+        response = self.request("user.update", {"id": 222, "member_number": None})
+        self.assert_status_code(response, 200)
+        self.assert_model_exists("user/222", {"member_number": None})
+
     def test_same_username(self) -> None:
         response = self.request("user.update", {"id": 1, "username": "admin"})
         self.assert_status_code(response, 200)