diff --git a/ofsc/models/__init__.py b/ofsc/models/__init__.py
index ebf86b5..91270fa 100644
--- a/ofsc/models/__init__.py
+++ b/ofsc/models/__init__.py
@@ -325,7 +325,7 @@ class BulkUpdateError(BaseModel):
 
 class BulkUpdateWarning(BaseModel):
     code: Optional[int] = None
-    message: Optional[int] = None
+    message: Optional[str] = None
 
 
 class BulkUpdateResult(BaseModel):
diff --git a/tests/test_model.py b/tests/test_model.py
index 68251c1..30fb88b 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -8,6 +8,7 @@
 from ofsc.models import (
     ActivityType,
     ActivityTypeGroup,
+    BulkUpdateResponse,
     CapacityArea,
     CapacityAreaListResponse,
     CapacityCategoryListResponse,
@@ -25,6 +26,26 @@
 )
 
 
+def test_bulk_update_response_accepts_string_warning_message():
+    raw = {
+        "results": [
+            {
+                "activityKeys": {"activityId": 4225, "apptNumber": "0010a000019oDas"},
+                "operationsPerformed": ["updateProperties", "updatePosition"],
+                "warnings": [
+                    {"code": 1, "message": "Activity moved to a non-scheduled bucket"},
+                ],
+            },
+        ],
+    }
+
+    response = BulkUpdateResponse.model_validate(raw)
+
+    warning = response.results[0].warnings[0]
+    assert warning.code == 1
+    assert warning.message == "Activity moved to a non-scheduled bucket"
+
+
 def test_translation_model_base():
     base = {"language": "en", "name": "Estimate", "languageISO": "en-US"}
     obj = Translation.model_validate(base)