diff --git a/simgear/package/ioapi.h b/simgear/package/ioapi.h
index 1dba776c..cbc0e33e 100755
--- a/simgear/package/ioapi.h
+++ b/simgear/package/ioapi.h
@@ -9,6 +9,11 @@
 #ifndef _ZLIBIOAPI_H
 #define _ZLIBIOAPI_H
 
+// Needed for some zlib installations:
+#ifndef OF
+  #define OF(x) x
+#endif
+
 
 #define ZLIB_FILEFUNC_SEEK_CUR (1)
 #define ZLIB_FILEFUNC_SEEK_END (2)