diff --git a/cubesatsim/cubesatsim.h b/cubesatsim/cubesatsim.h
index 59002b14..f6215d47 100644
--- a/cubesatsim/cubesatsim.h
+++ b/cubesatsim/cubesatsim.h
@@ -36,6 +36,7 @@
 #include <WiFi.h>
 #include "hardware/gpio.h"
 #include "hardware/adc.h"
+#include <ctype.h>
 
 // Pico GPIO pin assignments
 #define LPF_PIN 8  // LPF is installed