diff --git a/hardware/arduino/cores/arduino/HardwareSerial.cpp b/hardware/arduino/cores/arduino/HardwareSerial.cpp
index 1b1fa71d8..9985b78c6 100644
--- a/hardware/arduino/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/cores/arduino/HardwareSerial.cpp
@@ -46,8 +46,8 @@
 struct ring_buffer
 {
   unsigned char buffer[SERIAL_BUFFER_SIZE];
-  volatile int head;
-  volatile int tail;
+  volatile unsigned int head;
+  volatile unsigned int tail;
 };
 
 #if defined(USBCON)