diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index c03c714ce..3c7118f13 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -167,8 +167,8 @@ uint32_t tud_cdc_n_write_flush (uint8_t itf)
   uint16_t count = tu_fifo_read_n(&_cdcd_itf[itf].tx_ff, p_cdc->epin_buf, TU_ARRAY_SIZE(p_cdc->epin_buf));
   if ( count )
   {
-    TU_VERIFY( tud_cdc_n_connected(itf) ); // fifo is empty if not connected
-    TU_ASSERT( usbd_edpt_xfer(TUD_OPT_RHPORT, p_cdc->ep_in, p_cdc->epin_buf, count) );
+    TU_VERIFY( tud_cdc_n_connected(itf), 0 ); // fifo is empty if not connected
+    TU_ASSERT( usbd_edpt_xfer(TUD_OPT_RHPORT, p_cdc->ep_in, p_cdc->epin_buf, count), 0 );
   }
 
   return count;