diff --git a/pagure/exceptions.py b/pagure/exceptions.py
index 8764baf..e02abd6 100644
--- a/pagure/exceptions.py
+++ b/pagure/exceptions.py
@@ -68,3 +68,8 @@ class NoCorrespondingPR(PagureException):
     ''' Exception raised when no pull-request is found with the given
     information. '''
     pass
+
+
+class InvalidObjetException(PagureException):
+    ''' Exception raised when a given object is not what was expected. '''
+    pass
diff --git a/pagure/lib/__init__.py b/pagure/lib/__init__.py
index da2f3e9..1c9cd62 100644
--- a/pagure/lib/__init__.py
+++ b/pagure/lib/__init__.py
@@ -3219,6 +3219,10 @@ def set_watch_obj(session, user, obj, watch_status):
             pull_request_uid=obj.uid,
             watch=watch_status,
         )
+    else:
+        raise pagure.exceptions.InvalidObjetException(
+            'Unknow watch target: "%s"' % obj
+        )
 
     session.add(dbobj)