diff --git a/ports/stm32/usbd_cdc_interface.h b/ports/stm32/usbd_cdc_interface.h
index 6dcab7d4794517722fbae6cb609450cb1321ed69..98b8fc077d3997bcb9f86d082b4a5ca021fa84b5 100644
--- a/ports/stm32/usbd_cdc_interface.h
+++ b/ports/stm32/usbd_cdc_interface.h
@@ -43,8 +43,8 @@ typedef struct _usbd_cdc_itf_t {
     uint16_t rx_buf_get; // circular buffer index
 
     uint8_t tx_buf[USBD_CDC_TX_DATA_SIZE]; // data for USB IN endpoind is stored in this buffer
-    uint16_t tx_buf_ptr_in; // increment this pointer modulo APP_TX_DATA_SIZE when new data is available
-    volatile uint16_t tx_buf_ptr_out; // increment this pointer modulo APP_TX_DATA_SIZE when data is drained
+    uint16_t tx_buf_ptr_in; // increment this pointer modulo USBD_CDC_TX_DATA_SIZE when new data is available
+    volatile uint16_t tx_buf_ptr_out; // increment this pointer modulo USBD_CDC_TX_DATA_SIZE when data is drained
     uint16_t tx_buf_ptr_out_shadow; // shadow of above
     uint8_t tx_buf_ptr_wait_count; // used to implement a timeout waiting for low-level USB driver
     uint8_t tx_need_empty_packet; // used to flush the USB IN endpoint if the last packet was exactly the endpoint packet size