diff --git a/STM32F1/cores/maple/libmaple/usb/stm32f1/usb_cdcacm.c b/STM32F1/cores/maple/libmaple/usb/stm32f1/usb_cdcacm.c
index d4cd8b7..dbf82c1 100644
--- a/STM32F1/cores/maple/libmaple/usb/stm32f1/usb_cdcacm.c
+++ b/STM32F1/cores/maple/libmaple/usb/stm32f1/usb_cdcacm.c
@@ -458,7 +458,7 @@ uint8 usb_cdcacm_is_transmitting(void) {
 
 int usb_cdcacm_tx_available()
 {
-	return CDC_SERIAL_TX_BUFFER_SIZE - usb_cdcacm_get_pending();
+	return CDC_SERIAL_TX_BUFFER_SIZE - usb_cdcacm_get_pending() - 1;
 }
 
 uint16 usb_cdcacm_get_pending(void) {