diff --git a/armsrc/util.c b/armsrc/util.c
index 252b09d0d..89a5d598b 100644
--- a/armsrc/util.c
+++ b/armsrc/util.c
@@ -86,7 +86,7 @@ int hex2binarray(char *target, char *source) {
     return hex2binarray_n(target, source, strlen(source));
 }
 
-int hex2binarray_n(char *target, char *source, int sourcelen) {
+int hex2binarray_n(char *target, const char *source, int sourcelen) {
     int count = 0;
 
     // process 4 bits (1 hex digit) at a time
diff --git a/armsrc/util.h b/armsrc/util.h
index 5099433e5..8842b5d8d 100644
--- a/armsrc/util.h
+++ b/armsrc/util.h
@@ -85,7 +85,7 @@ size_t nbytes(size_t nbits);
 uint8_t hex2int(char x);
 
 int hex2binarray(char *target, char *source);
-int hex2binarray_n(char *target, char *source, int sourcelen);
+int hex2binarray_n(char *target, const char *source, int sourcelen);
 int binarray2hex(const uint8_t *bs, int bs_len, uint8_t *hex);
 
 void LED(int led, int ms);