diff --git a/src/documents/serialisers.py b/src/documents/serialisers.py
index 1a491c5a4..e47f42e09 100644
--- a/src/documents/serialisers.py
+++ b/src/documents/serialisers.py
@@ -3213,6 +3213,13 @@ class WorkflowActionSerializer(serializers.ModelSerializer[WorkflowAction]):
                         {"assign_title": f'Invalid f-string detected: "{e.args[0]}"'},
                     )
 
+        if attrs.get("assign_custom_fields_values"):
+            # Empty strings treated as None to avoid unexpected behavior
+            attrs["assign_custom_fields_values"] = {
+                field_id: (None if value == "" else value)
+                for field_id, value in attrs["assign_custom_fields_values"].items()
+            }
+
         if (
             "type" in attrs
             and attrs["type"] == WorkflowAction.WorkflowActionType.EMAIL
diff --git a/src/documents/tests/test_api_workflows.py b/src/documents/tests/test_api_workflows.py
index cdd3a5f40..e2689001a 100644
--- a/src/documents/tests/test_api_workflows.py
+++ b/src/documents/tests/test_api_workflows.py
@@ -422,6 +422,11 @@ class TestApiWorkflows(DirectoriesMixin, APITestCase):
             json.dumps(
                 {
                     "assign_title": "",
+                    "assign_custom_fields": [self.cf1.id, self.cf2.id],
+                    "assign_custom_fields_values": {
+                        str(self.cf1.id): "",
+                        str(self.cf2.id): 0,
+                    },
                 },
             ),
             content_type="application/json",
@@ -429,6 +434,10 @@ class TestApiWorkflows(DirectoriesMixin, APITestCase):
         self.assertEqual(response.status_code, status.HTTP_201_CREATED)
         action = WorkflowAction.objects.get(id=response.data["id"])
         self.assertIsNone(action.assign_title)
+        self.assertEqual(
+            action.assign_custom_fields_values,
+            {str(self.cf1.id): None, str(self.cf2.id): 0},
+        )
 
         response = self.client.post(
             self.ENDPOINT_TRIGGERS,