diff --git a/sys/vfs/hammer/hammer_vnops.c b/sys/vfs/hammer/hammer_vnops.c
index c8150ed..911a074 100644
--- a/sys/vfs/hammer/hammer_vnops.c
+++ b/sys/vfs/hammer/hammer_vnops.c
@@ -2577,7 +2577,7 @@ hammer_vop_mountctl(struct vop_mountctl_args *ap)
 						    &error);
 		}
 
-		*ap->a_res += usedbytes;
+		*ap->a_res = usedbytes;
 		break;
 	default:
 		error = vop_stdmountctl(ap);