diff --git a/src/utils/str.py b/src/utils/str.py
index a227ff7dc..f3003b43a 100644
--- a/src/utils/str.py
+++ b/src/utils/str.py
@@ -448,6 +448,13 @@ def format(s, *args, **kwargs):
     # to add the character to the _formatRe regexp or it will be ignored
     # (and hard to debug if you don't know the trick).
     # Of course, you should also document it in the docstring above.
+    if sys.version_info.major < 3:
+        def pred(s):
+            if isinstance(s, unicode):
+                return s.encode('utf8')
+            else:
+                return s
+        args = map(pred, args)
     args = list(args)
     args.reverse() # For more efficient popping.
     def sub(match):