diff --git a/payloads/libpayload/include/string.h b/payloads/libpayload/include/string.h
index 4aff0e8994..52379a034d 100644
--- a/payloads/libpayload/include/string.h
+++ b/payloads/libpayload/include/string.h
@@ -73,7 +73,7 @@ char *strerror(int errnum);
  * @defgroup string Unicode functions
  * @{
  */
-char *utf16le_to_ascii(uint16_t *utf16_string, int maxlen);
+char *utf16le_to_ascii(const uint16_t *utf16_string, size_t maxlen);
 /** @} */
 
 /**
diff --git a/payloads/libpayload/libc/string.c b/payloads/libpayload/libc/string.c
index 9a5a1ea4c1..fd88a88ad2 100644
--- a/payloads/libpayload/libc/string.c
+++ b/payloads/libpayload/libc/string.c
@@ -645,12 +645,11 @@ char *strerror(int errnum)
  * @param maxlen Maximum possible length of the string in code points
  * @return Newly allocated ASCII string
  */
-char *utf16le_to_ascii(uint16_t *utf16_string, int maxlen)
+char *utf16le_to_ascii(const uint16_t *utf16_string, size_t maxlen)
 {
 	char *ascii_string = xmalloc(maxlen + 1);  /* +1 for trailing \0 */
 	ascii_string[maxlen] = '\0';
-	int i;
-	for (i = 0; i < maxlen; i++) {
+	for (size_t i = 0; i < maxlen; i++) {
 		uint16_t wchar = utf16_string[i];
 		ascii_string[i] = wchar > 0x7f ? '?' : (char)wchar;
 	}