diff --git a/generator/lua.ml b/generator/lua.ml
index 8e735c99a..188490a65 100644
--- a/generator/lua.ml
+++ b/generator/lua.ml
@@ -49,8 +49,10 @@ let generate_lua_c () =
 #include <lauxlib.h>
 
 #if LUA_VERSION_NUM >= 502
+#ifndef lua_objlen
 #define lua_objlen lua_rawlen
 #endif
+#endif
 
 #include <guestfs.h>
 #include \"guestfs-internal-frontend.h\"