diff --git a/sphinx/util/osutil.py b/sphinx/util/osutil.py
index cd9aba7e6..a78eeb98c 100644
--- a/sphinx/util/osutil.py
+++ b/sphinx/util/osutil.py
@@ -288,7 +288,7 @@ class FileAvoidWrite(object):
         # Proxy to _io instance.
         if not self._io:
             raise Exception('Must write to FileAvoidWrite before other '
-                'methods can be used')
+                            'methods can be used')
 
         return getattr(self._io, name)