diff --git a/polynum_blackboard/ws.py b/polynum_blackboard/ws.py
index 5f65627..40c2fe0 100644
--- a/polynum_blackboard/ws.py
+++ b/polynum_blackboard/ws.py
@@ -108,7 +108,7 @@ class BlackboardConnector(object):
             open_to_visitors=True, auto_inscription=True, password=None):
         assert isinstance(course_id, unicode) and course_id
         assert isinstance(course_name, unicode) and course_name
-        assert isinstance(category, unicode) and unicode
+        assert isinstance(category, unicode) and category
 
         open_to_visitors = bool2str(open_to_visitors)
         auto_inscription = bool2str(auto_inscription)