diff --git a/box.c b/box.c
index 54bd5c6258..616a8acf7d 100644
--- a/box.c
+++ b/box.c
@@ -759,6 +759,7 @@ rb_box_cleanup_local_extension(VALUE cleanup)
 #ifndef _WIN32
     if (p) box_ext_cleanup_free(p);
 #endif
+    (void)p;
 }
 
 static int