diff --git a/unzip/ioapi.h b/unzip/ioapi.h
index 1dba776c..75171b9b 100644
--- a/unzip/ioapi.h
+++ b/unzip/ioapi.h
@@ -21,6 +21,9 @@
 #define ZLIB_FILEFUNC_MODE_EXISTING (4)
 #define ZLIB_FILEFUNC_MODE_CREATE   (8)
 
+#ifndef OF
+#define OF(x) x
+#endif
 
 #ifndef ZCALLBACK