diff --git a/src/vimio.h b/src/vimio.h
index 71a52cd..a4dd030 100644
--- a/src/vimio.h
+++ b/src/vimio.h
@@ -13,4 +13,7 @@
 # define _CRT_NONSTDC_NO_DEPRECATE
 #endif
 
-#include <io.h>
+/* cproto fails on missing include files */
+#ifndef PROTO
+# include <io.h>
+#endif