diff --git a/Sources/unzip/ioapi.h b/Sources/unzip/ioapi.h
index 7d457baab..2b0e0d79b 100644
--- a/Sources/unzip/ioapi.h
+++ b/Sources/unzip/ioapi.h
@@ -9,6 +9,11 @@
 #ifndef _ZLIBIOAPI_H
 #define _ZLIBIOAPI_H
 
+/* Gentoo removed OF from their copy of zconf.h (https://bugs.gentoo.org/show_bug.cgi?id=383179) */
+/* but our copy of minizip needs it. */
+#ifndef OF
+#define OF(args) args
+#endif
 
 #define ZLIB_FILEFUNC_SEEK_CUR (1)
 #define ZLIB_FILEFUNC_SEEK_END (2)