diff --git a/hardware/arduino/avr/cores/arduino/CDC.cpp b/hardware/arduino/avr/cores/arduino/CDC.cpp
index 1797ed67c..bf3d07603 100644
--- a/hardware/arduino/avr/cores/arduino/CDC.cpp
+++ b/hardware/arduino/avr/cores/arduino/CDC.cpp
@@ -116,10 +116,12 @@ bool WEAK CDC_Setup(Setup& setup)
 
 void Serial_::begin(unsigned long /* baud_count */)
 {
+	peek_buffer = -1;
 }
 
 void Serial_::begin(unsigned long /* baud_count */, byte /* config */)
 {
+	peek_buffer = -1;
 }
 
 void Serial_::end(void)
diff --git a/hardware/arduino/avr/cores/arduino/USBAPI.h b/hardware/arduino/avr/cores/arduino/USBAPI.h
index 3b613d423..7d41ca05b 100644
--- a/hardware/arduino/avr/cores/arduino/USBAPI.h
+++ b/hardware/arduino/avr/cores/arduino/USBAPI.h
@@ -36,8 +36,9 @@ struct ring_buffer;
 class Serial_ : public Stream
 {
 private:
-	int peek_buffer = -1;
+	int peek_buffer;
 public:
+	Serial_() { peek_buffer = -1; };
 	void begin(unsigned long);
 	void begin(unsigned long, uint8_t);
 	void end(void);