diff --git a/cores/arduino/USBAPI.h b/cores/arduino/USBAPI.h
index eb2e593..1a4183b 100644
--- a/cores/arduino/USBAPI.h
+++ b/cores/arduino/USBAPI.h
@@ -25,6 +25,8 @@ extern USBDevice_ USBDevice;
 //================================================================================
 //	Serial over CDC (Serial1 is the physical port)
 
+struct ring_buffer;
+
 class Serial_ : public Stream
 {
 private:
@@ -193,4 +195,4 @@ void USB_Flush(uint8_t ep);
 
 #endif
 
-#endif /* if defined(USBCON) */
\ No newline at end of file
+#endif /* if defined(USBCON) */