diff --git a/sys/ufs/ffs/ffs_inode.c b/sys/ufs/ffs/ffs_inode.c
index f8584d539545..4571a1cdc92b 100644
--- a/sys/ufs/ffs/ffs_inode.c
+++ b/sys/ufs/ffs/ffs_inode.c
@@ -546,9 +546,9 @@ ffs_truncate(vp, length, flags, cred)
 	 */
 	ip->i_size = length;
 	DIP_SET(ip, i_size, length);
-	DIP_SET(ip, i_blocks, DIP(ip, i_blocks) - blocksreleased);
-
-	if (DIP(ip, i_blocks) < 0)			/* sanity */
+	if (DIP(ip, i_blocks) >= blocksreleased)
+		DIP_SET(ip, i_blocks, DIP(ip, i_blocks) - blocksreleased);
+	else	/* sanity */
 		DIP_SET(ip, i_blocks, 0);
 	ip->i_flag |= IN_CHANGE;
 #ifdef QUOTA