Index: Modules/_ctypes/cfield.c
===================================================================
--- Modules/_ctypes/cfield.c	(revision 74022)
+++ Modules/_ctypes/cfield.c	(working copy)
@@ -428,7 +428,7 @@
 
 /* This seems nore a compiler issue than a Windows/non-Windows one */
 #ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
+#  define BIT_MASK(size) ((1I64 << NUM_BITS(size))-1)
 #else
 #  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
 #endif