diff --git a/disco/bot/parser.py b/disco/bot/parser.py
index 4f7d267..8283fc8 100644
--- a/disco/bot/parser.py
+++ b/disco/bot/parser.py
@@ -131,6 +131,7 @@ class ArgumentSet(object):
         value : str
             The string value to attempt conversion on.
         """
+        exc = None
         for typ_name in types:
             typ = self.types.get(typ_name)
             if not typ: