diff --git a/Lib/string.py b/Lib/string.py
index 2a450cd..d4fee39 100644
--- a/Lib/string.py
+++ b/Lib/string.py
@@ -145,24 +145,18 @@
             mapping = args[0]
         # Helper function for .sub()
         def convert(mo):
-            named = mo.group('named')
+            named = mo.group('named') or mo.group('braced')
             if named is not None:
                 try:
                     # We use this idiom instead of str() because the latter
                     # will fail if val is a Unicode containing non-ASCII
                     return '%s' % (mapping[named],)
                 except KeyError:
-                    return self.delimiter + named
-            braced = mo.group('braced')
-            if braced is not None:
-                try:
-                    return '%s' % (mapping[braced],)
-                except KeyError:
-                    return self.delimiter + '{' + braced + '}'
+                    return mo.group()
             if mo.group('escaped') is not None:
                 return self.delimiter
             if mo.group('invalid') is not None:
-                return self.delimiter
+                return mo.group()
             raise ValueError('Unrecognized named group in pattern',
                              self.pattern)
         return self.pattern.sub(convert, self.template)