diff --git a/runtime/flang/initpar.c b/runtime/flang/initpar.c
index 34e3b02251bc..853c88836e5f 100644
--- a/runtime/flang/initpar.c
+++ b/runtime/flang/initpar.c
@@ -35,7 +35,7 @@
 
 #if   defined(TARGET_OSX)
 #include <crt_externs.h>
-#elif defined(__WIN64)
+#elif defined(_WIN64)
 /* OPENTOOLS14 has changed the name.  wrap _environ for all of windowws */
 char **__io_environ();
 #else