diff --git a/sys/kern/vfs_cluster.c b/sys/kern/vfs_cluster.c
index aca7058e8f1c..19ee05f9d702 100644
--- a/sys/kern/vfs_cluster.c
+++ b/sys/kern/vfs_cluster.c
@@ -315,7 +315,7 @@ cluster_rbuild(vp, filesize, lbn, blkno, size, run, fbp)
 	int i, inc, j, toff;
 
 	KASSERT(size == vp->v_mount->mnt_stat.f_iosize,
-	    ("cluster_rbuild: size %ld != filesize %jd\n",
+	    ("cluster_rbuild: size %ld != f_iosize %jd\n",
 	    size, (intmax_t)vp->v_mount->mnt_stat.f_iosize));
 
 	/*