diff --git a/src/python/oftest/controller.py b/src/python/oftest/controller.py
index bee9ab9..df24c89 100644
--- a/src/python/oftest/controller.py
+++ b/src/python/oftest/controller.py
@@ -588,7 +588,7 @@
         """
 
         exp_msg_str = "unspecified"
-        if exp_msg:
+        if exp_msg is not None:
             exp_msg_str = ofp.ofp_type_map.get(exp_msg, "unknown (%d)" % 
                                                exp_msg)