diff --git a/res/res_musiconhold.c b/res/res_musiconhold.c
index 58894a1dc8613c404f6f0a3f01d2648ebfa20f2d..c53bf29c1f055249d190ad27fd76a4da8b577bd3 100644
--- a/res/res_musiconhold.c
+++ b/res/res_musiconhold.c
@@ -187,7 +187,7 @@ static struct ao2_container *mohclasses;
 static int reload(void);
 
 #define mohclass_ref(class)   (ao2_ref((class), +1), class)
-#define mohclass_unref(class) (ao2_ref((class), -1), NULL)
+#define mohclass_unref(class) (ao2_ref((class), -1), (struct mohclass *) NULL)
 
 static void moh_files_release(struct ast_channel *chan, void *data)
 {