Index: Modules/_ctypes/cfield.c
===================================================================
--- Modules/_ctypes/cfield.c	(revision 85158)
+++ Modules/_ctypes/cfield.c	(working copy)
@@ -428,13 +428,15 @@
 #define LOW_BIT(x)  ((x) & 0xFFFF)
 #define NUM_BITS(x) ((x) >> 16)
 
-/* This seems nore a compiler issue than a Windows/non-Windows one */
-#ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
+#ifdef HAVE_LONG_LONG
+#  define MAX_SIZE_INT(n) Py_ULL(1)
 #else
-#  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
+#  define MAX_SIZE_INT(n) n##UL
 #endif
 
+/* BIT_MASK(0) won't return correct result */
+#define BIT_MASK(size) ((((MAX_SIZE_INT(1) << (NUM_BITS(size) - 1)) - 1) << 1) + 1)
+
 /* This macro CHANGES the first parameter IN PLACE. For proper sign handling,
    we must first shift left, then right.
 */