diff --git a/drivers/serial/ptmx.c b/drivers/serial/ptmx.c
index cf372e0a65..05548df46c 100644
--- a/drivers/serial/ptmx.c
+++ b/drivers/serial/ptmx.c
@@ -220,7 +220,7 @@ static int ptmx_open(FAR struct file *filep)
    */
 
   ret = unregister_driver(devname);
-  DEBUGASSERT(ret >= 0);  /* unregister_driver() should never fail */
+  DEBUGASSERT(ret >= 0 || ret == -EBUSY);  /* unregister_driver() should never fail */
 
   nxsem_post(&g_ptmx.px_exclsem);
   return OK;