diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index fa4c0f8f62361a..ad64622c0be974 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -609,7 +609,7 @@ def login(self, user, password):
         """
         typ, dat = self._simple_command('LOGIN', user, self._quote(password))
         if typ != 'OK':
-            raise self.error(dat[-1])
+            raise self.error(dat[-1].decode('UTF-8', 'replace'))
         self.state = 'AUTH'
         return typ, dat