diff --git a/src/saml2/attribute_converter.py b/src/saml2/attribute_converter.py
index c0aaf33..4888db3 100644
--- a/src/saml2/attribute_converter.py
+++ b/src/saml2/attribute_converter.py
@@ -428,7 +428,7 @@ class AttributeConverter(object):
             _attr = self._to[attr]
         except KeyError:
             try:
-                _attr = self._to[attr.to_lower()]
+                _attr = self._to[attr.lower()]
             except:
                 _attr = ''