diff --git a/channel/channel_factory.py b/channel/channel_factory.py
index 186d445..7b53e80 100644
--- a/channel/channel_factory.py
+++ b/channel/channel_factory.py
@@ -46,4 +46,4 @@ def create_channel(channel_type):
         return HttpChannel()
 
     else:
-        raise RuntimeError
+        raise RuntimeError("unknown channel_type in config.json: " + channel_type)