diff --git a/hardware/arduino/cores/arduino/HardwareSerial.cpp b/hardware/arduino/cores/arduino/HardwareSerial.cpp
index 4397efb7e..38e87c49f 100644
--- a/hardware/arduino/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/cores/arduino/HardwareSerial.cpp
@@ -191,7 +191,7 @@ HardwareSerial::HardwareSerial(ring_buffer *rx_buffer,
 
 // Public Methods //////////////////////////////////////////////////////////////
 
-void HardwareSerial::begin(long baud)
+void HardwareSerial::begin(unsigned long baud)
 {
   uint16_t baud_setting;
   bool use_u2x = true;
diff --git a/hardware/arduino/cores/arduino/HardwareSerial.h b/hardware/arduino/cores/arduino/HardwareSerial.h
index 3efa775f8..9dc67c4c9 100644
--- a/hardware/arduino/cores/arduino/HardwareSerial.h
+++ b/hardware/arduino/cores/arduino/HardwareSerial.h
@@ -48,7 +48,7 @@ class HardwareSerial : public Stream
       volatile uint8_t *ucsra, volatile uint8_t *ucsrb,
       volatile uint8_t *udr,
       uint8_t rxen, uint8_t txen, uint8_t rxcie, uint8_t udre, uint8_t u2x);
-    void begin(long);
+    void begin(unsigned long);
     void end();
     virtual int available(void);
     virtual int peek(void);