diff --git a/STM32F1/cores/maple/usb_serial.h b/STM32F1/cores/maple/usb_serial.h
index d6c3e02..153eb8e 100644
--- a/STM32F1/cores/maple/usb_serial.h
+++ b/STM32F1/cores/maple/usb_serial.h
@@ -51,6 +51,7 @@ public:
 
     virtual int available(void);// Changed to virtual
 
+    size_t usb_serial_class::readBytes(char *buf, const size_t& len);
     uint32 read(uint8 * buf, uint32 len);
     // uint8  read(void);