diff --git a/wlauto/utils/types.py b/wlauto/utils/types.py
index f4fe9cb6..a03cf9eb 100644
--- a/wlauto/utils/types.py
+++ b/wlauto/utils/types.py
@@ -243,3 +243,6 @@ class caseless_string(str):
         if isinstance(basestring, other):
             other = other.lower()
         return cmp(self.lower(), other)
+
+    def format(self, *args, **kwargs):
+        return caseless_string(super(caseless_string, self).format(*args, **kwargs))