--- a/server/sqlutils.py	Thu Oct 24 18:28:18 2013 +0200
+++ b/server/sqlutils.py	Thu Nov 28 18:55:24 2013 +0100
@@ -337,7 +337,8 @@
             if value is not None:
                 self.values.append(value)
         def finalize(self):
-            return ', '.join(self.values)
+            return ', '.join(str(v) for v in self.values)
+
     cnx.create_aggregate("GROUP_CONCAT", 1, group_concat)
 
     def _limit_size(text, maxsize, format='text/plain'):