diff --git a/src/oic/oauth2/message.py b/src/oic/oauth2/message.py
index 48dbde362..61b364b9d 100644
--- a/src/oic/oauth2/message.py
+++ b/src/oic/oauth2/message.py
@@ -319,7 +319,7 @@ def to_dict(self, lev=0):
 
             if isinstance(val, Message):
                 _res[key] = val.to_dict(lev + 1)
-            elif isinstance(val, list) and isinstance(val[0], Message):
+            elif isinstance(val, list) and isinstance(next(iter(val or []), None), Message):
                 _res[key] = [v.to_dict(lev) for v in val]
             else:
                 _res[key] = val