Index: head/sys/kern/vfs_bio.c
===================================================================
--- head/sys/kern/vfs_bio.c
+++ head/sys/kern/vfs_bio.c
@@ -4927,9 +4927,12 @@
 	db_printf("b_io_tracking: b_io_tcnt = %u\n", bp->b_io_tcnt);
 
 	i = bp->b_io_tcnt % BUF_TRACKING_SIZE;
-	for (j = 1; j <= BUF_TRACKING_SIZE; j++)
+	for (j = 1; j <= BUF_TRACKING_SIZE; j++) {
+		if (bp->b_io_tracking[BUF_TRACKING_ENTRY(i - j)] == NULL)
+			continue;
 		db_printf(" %2u: %s\n", j,
 		    bp->b_io_tracking[BUF_TRACKING_ENTRY(i - j)]);
+	}
 #elif defined(BUF_TRACKING)
 	db_printf("b_io_tracking: %s\n", bp->b_io_tracking);
 #endif