diff --git a/tommath.h b/tommath.h
index 5523ad0..0119906 100644
--- a/tommath.h
+++ b/tommath.h
@@ -27,7 +27,7 @@ extern "C" {
 #endif
 
 /* MS Visual C++ doesn't have a 128bit type for words, so fall back to 32bit MPI's (where words are 64bit) */
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) || defined(__LLP64__)
 #   define MP_32BIT
 #endif