diff --git a/flask/debughelpers.py b/flask/debughelpers.py
index 504fab93..f3bac185 100644
--- a/flask/debughelpers.py
+++ b/flask/debughelpers.py
@@ -8,6 +8,7 @@
     :copyright: (c) 2011 by Armin Ronacher.
     :license: BSD, see LICENSE for more details.
 """
+from flask._compat import implements_to_string
 
 
 class UnexpectedUnicodeError(AssertionError, UnicodeError):
@@ -16,6 +17,7 @@ class UnexpectedUnicodeError(AssertionError, UnicodeError):
     """
 
 
+@implements_to_string
 class DebugFilesKeyError(KeyError, AssertionError):
     """Raised from request.files during debugging.  The idea is that it can
     provide a better error message than just a generic KeyError/BadRequest.
@@ -33,7 +35,7 @@ class DebugFilesKeyError(KeyError, AssertionError):
             buf.append('\n\nThe browser instead transmitted some file names. '
                        'This was submitted: %s' % ', '.join('"%s"' % x
                             for x in form_matches))
-        self.msg = ''.join(buf).encode('utf-8')
+        self.msg = ''.join(buf)
 
     def __str__(self):
         return self.msg