diff --git a/src/apdu.h b/src/apdu.h
index d8533c8..8e594b0 100644
--- a/src/apdu.h
+++ b/src/apdu.h
@@ -20,6 +20,7 @@
 
 #include <stdlib.h>
 #include "pico/stdlib.h"
+#include <stdio.h>
 
 typedef struct app {
     const uint8_t *aid;