diff --git a/library/sha512.c b/library/sha512.c
index 5decc8f..2c22259 100644
--- a/library/sha512.c
+++ b/library/sha512.c
@@ -35,6 +35,12 @@
 
 #include "polarssl/sha512.h"
 
+#if defined(_MSC_VER) || defined(__WATCOMC__)
+  #define UL64(x) x##ui64
+#else
+  #define UL64(x) x##ULL
+#endif
+
 #include <string.h>
 
 #if defined(POLARSSL_FS_IO)