--- a/sys/vfs/ufs/ffs_alloc.c	2026-06-29 12:51:19.000000000 +0000
+++ b/sys/vfs/ufs/ffs_alloc.c	2026-10-26 03:28:07.221962165 +0000
@@ -428,7 +428,8 @@
 		ssize = len;
 	} else {
 #ifdef DIAGNOSTIC
-		if (start_ap[start_lvl-1].in_lbn == idp->in_lbn)
+		if (start_lvl > 0 &&
+		    start_ap[start_lvl-1].in_lbn == idp->in_lbn)
 			panic("ffs_reallocblk: start == end");
 #endif
 		ssize = len - (idp->in_off + 1);