diff --git a/sbin/newfs/mkfs.c b/sbin/newfs/mkfs.c
index 4ca25a23f495..062a82d66c5b 100644
--- a/sbin/newfs/mkfs.c
+++ b/sbin/newfs/mkfs.c
@@ -284,6 +284,8 @@ mkfs(pp, fsys, fi, fo)
 		sizepb *= NINDIR(&sblock);
 		sblock.fs_maxfilesize += sizepb;
 	}
+	/* XXX - hack to prevent overflow of a 32bit block number */
+	sblock.fs_maxfilesize = MIN(sblock.fs_maxfilesize, (u_quad_t) 1 << 39);
 	/*
 	 * Validate specified/determined secpercyl
 	 * and calculate minimum cylinders per group.