diff --git a/hardware/arduino/cores/arduino/USBAPI.h b/hardware/arduino/cores/arduino/USBAPI.h
index d506b585f..753bc6b20 100644
--- a/hardware/arduino/cores/arduino/USBAPI.h
+++ b/hardware/arduino/cores/arduino/USBAPI.h
@@ -28,7 +28,7 @@ extern USBDevice_ USBDevice;
 class Serial_ : public Stream
 {
 private:
-	int peek_buffer;
+	int peek_buffer = -1;
 public:
 	void begin(unsigned long);
 	void begin(unsigned long, uint8_t);