diff -r 19a683a0047c -r 5def1d98fce7 schema.py
--- a/schema.py	Thu Aug 28 18:29:14 2014 +0200
+++ b/schema.py	Fri Aug 29 07:44:13 2014 +0200
@@ -125,7 +125,8 @@
     if 'U' in defined:
         mainvars.add('U')
     if not mainvars:
-        raise Exception('unable to guess selection variables')
+        raise BadSchemaDefinition('unable to guess selection variables in %r'
+                                  % expression)
     return mainvars
 
 def split_expression(rqlstring):