diff --git a/lib/sqlalchemy/orm/evaluator.py b/lib/sqlalchemy/orm/evaluator.py
index c4517e4947..3469a24c8c 100644
--- a/lib/sqlalchemy/orm/evaluator.py
+++ b/lib/sqlalchemy/orm/evaluator.py
@@ -78,6 +78,8 @@ class EvaluatorCompiler(object):
                 if left_val is None or right_val is None:
                     return None
                 return operator(eval_left(obj), eval_right(obj))
+        else:
+            raise UnevaluatableError("Cannot evaluate %s with operator %s" % (type(clause).__name__, clause.operator))
         return evaluate
 
     def visit_unary(self, clause):