diff --git a/src/if_ruby.c b/src/if_ruby.c
index 8b1590e..96d9453 100644
--- a/src/if_ruby.c
+++ b/src/if_ruby.c
@@ -189,7 +189,7 @@
 #endif
 
 #if defined(DYNAMIC_RUBY) || defined(PROTO)
-# ifdef PROTO
+# if defined(PROTO) && !defined(HINSTANCE)
 #  define HINSTANCE int		/* for generating prototypes */
 # endif