--- a/mercurial/util_win32.py
+++ b/mercurial/util_win32.py
@@ -209,9 +209,9 @@ class posixfile_nt(object):
 
     def __init__(self, name, mode='rb'):
         access = 0
-        if 'r' in mode or '+' in mode:
+        if 'r' in mode:
             access |= win32file.GENERIC_READ
-        if 'w' in mode or 'a' in mode:
+        if 'w' in mode or 'a' in mode or '+' in mode:
             access |= win32file.GENERIC_WRITE
         if 'r' in mode:
             creation = win32file.OPEN_EXISTING