diff --git a/warden3/warden_server/warden_server.py b/warden3/warden_server/warden_server.py
index ca7e4f4322323a0fe496f6838a09c1fdd3f3cca9..a392756cfcf7ab53df780c8cb6afe7bde913bcb1 100755
--- a/warden3/warden_server/warden_server.py
+++ b/warden3/warden_server/warden_server.py
@@ -538,7 +538,7 @@ class MySQL(ObjectReq):
             lastid = self.crs.lastrowid
 
             catlist = event.get('Category', ["Other"])
-            cats = set(catlist) | {cat.split(".", 1)[0] for cat in catlist}
+            cats = set(catlist) | set(cat.split(".", 1)[0] for cat in catlist)
             for cat in cats:
                 cat_id = self.catmap.get(cat, self.catmap_other)
                 self.crs.execute("INSERT INTO event_category_mapping (event_id,category_id) VALUES (%s, %s)", (lastid, cat_id))