diff --git a/epicardium/epicardium.h b/epicardium/epicardium.h
index 13358966d499dcd430f3ff6dcca4df9165e48311..400ca00eb80141bfc2838709d47a5ea779fc5078 100644
--- a/epicardium/epicardium.h
+++ b/epicardium/epicardium.h
@@ -317,7 +317,7 @@ API(API_THERMISTOR_VOLTAGE, int epic_read_thermistor_voltage(float *result));
  * :param length:  Amount of bytes to print.
  */
 API(API_UART_WRITE_STR, void epic_uart_write_str(
-	const char *str, intptr_t length
+	const char *str, size_t length
 ));
 
 /**
diff --git a/epicardium/modules/serial.c b/epicardium/modules/serial.c
index ef5b65edda24b9ce828afcec18cb84f2ff7f27ef..befca3b50aa00c0a6bda4e0a7cbecd13ea8420c0 100644
--- a/epicardium/modules/serial.c
+++ b/epicardium/modules/serial.c
@@ -54,7 +54,7 @@ void serial_return_to_synchronous()
 /*
  * API-call to write a string.  Output goes to both CDCACM and UART
  */
-void epic_uart_write_str(const char *str, intptr_t length)
+void epic_uart_write_str(const char *str, size_t length)
 {
 	if (length == 0) {
 		return;