diff --git a/glm/core/_detail.hpp b/glm/core/_detail.hpp
index 0748a83d..4d6a57ed 100644
--- a/glm/core/_detail.hpp
+++ b/glm/core/_detail.hpp
@@ -18,7 +18,7 @@ namespace detail
 {
 	class thalf;
 
-#if(__STDC_VERSION__ && (__STDC_VERSION__ >= 199901L)) // C99 detected, 64 bit types available
+#if(defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)) // C99 detected, 64 bit types available
 	typedef int64_t								sint64;
 	typedef uint64_t							uint64;
 #elif(GLM_COMPILER & GLM_COMPILER_VC)