diff --git a/bibble.py b/bibble.py
index dfd42dd..1bbd0d6 100644
--- a/bibble.py
+++ b/bibble.py
@@ -87,6 +87,7 @@ def _extra_urls(entry):
     """
     urls = {}
     for k, v in entry.fields.iteritems():
+        k = k.lower()
         if not k.endswith('_url'):
             continue
         k = k[:-4]