diff --git a/lib/env.h b/lib/env.h
index d5c2fdb7..bb0acf8e 100644
--- a/lib/env.h
+++ b/lib/env.h
@@ -46,12 +46,18 @@
 #if defined (__x86_64__)
 // This also works for the x32 ABI, which has a 64-bit word size.
 #  define BASE64_WORDSIZE 64
-#elif defined (_INTEGRAL_MAX_BITS)
-#  define BASE64_WORDSIZE _INTEGRAL_MAX_BITS
 #elif defined (__WORDSIZE)
 #  define BASE64_WORDSIZE __WORDSIZE
 #elif defined (__SIZE_WIDTH__)
 #  define BASE64_WORDSIZE __SIZE_WIDTH__
+#elif defined (_WIN32)
+// _WIN32 is defined for all Windows targets but _WIN64 is only defined for
+// 64-bit targets.
+#  if defined (_WIN64)
+#    define BASE64_WORDSIZE 64
+#  else
+#    define BASE64_WORDSIZE 32
+#  endif
 #else
 #  error BASE64_WORDSIZE_NOT_DEFINED
 #endif