diff --git a/src/parser.h b/src/parser.h
index dac634d..21e9170 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -23,7 +23,9 @@
 #include "tree_internal.h"
 
 #ifdef __APPLE__
-#define MAP_ANONYMOUS MAP_ANON
+# ifndef MAP_ANONYMOUS
+#  define MAP_ANONYMOUS MAP_ANON
+# endif
 #endif
 
 /**