diff --git a/firmware/lux.cpp b/firmware/lux.cpp
index 1f1df37..f1328a4 100644
--- a/firmware/lux.cpp
+++ b/firmware/lux.cpp
@@ -4,6 +4,7 @@
 
 #include "lux.h"
 
+#include <stdio.h>
 #include <string.h>
 #include <FreeRTOS.h>
 #include <task.h>
@@ -44,7 +45,7 @@ extern "C" void lux_task(void *pvParameters) {
         for(volatile int i = 0; i < 100; i++);
         spi_read(1, 0x00, buf, 2, SPI_8BIT);
 
-        char hex[5];
+        char hex[5] = {0};
         int_to_hex(buf[0], hex);
         int_to_hex(buf[1], hex + 2);
         hex[4] = '\0';