diff --git a/sys/dev/disk/nata/atapi-cd.c b/sys/dev/disk/nata/atapi-cd.c
index c9b8162..997b7b8 100644
--- a/sys/dev/disk/nata/atapi-cd.c
+++ b/sys/dev/disk/nata/atapi-cd.c
@@ -1678,6 +1678,9 @@ acd_prevent_allow(device_t dev, int lock)
     int8_t ccb[16] = { ATAPI_PREVENT_ALLOW, 0, 0, 0, lock,
 		       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
 
+    kprintf("acd_prevent_allow: lock=%d\n", lock);
+    print_backtrace(-1);
+
     return ata_atapicmd(dev, ccb, NULL, 0, 0, 30);
 }