diff --git a/include/ucore/string.h b/include/ucore/string.h
index eb5e3ee..4225a2a 100644
--- a/include/ucore/string.h
+++ b/include/ucore/string.h
@@ -36,7 +36,7 @@ uint8_t* uc_hex_decode(const char *in, size_t maxlen,uint8_t *out);
 /** As uc_hex_encode , but inserts the @delim string between each converted
  *  byte (each 2 hex chars)
  */
-char* uc_hex_encode_delim(const uint8_t *in, size_t inlen, char *out, int outlen, char *delim);
+char* uc_hex_encode_delim(const uint8_t *in, size_t inlen, char *out, int outlen, const char *delim);
 
 /**
  * Convert the string to lowercase, calls tolower() on each char.
diff --git a/src/hex.c b/src/hex.c
index e361856..b2a162d 100644
--- a/src/hex.c
+++ b/src/hex.c
@@ -28,14 +28,14 @@ uc_hex_encode(const uint8_t *in, size_t len, char *out)
 }
 
 char*
-uc_hex_encode_delim(const uint8_t *in, size_t inlen, char *out, int outlen, char *delim)
+uc_hex_encode_delim(const uint8_t *in, size_t inlen, char *out, int outlen, const char *delim)
 {
     size_t i;
     char *outp = out;
 
     for (i = 0; i < inlen; i++) {
         unsigned int hn, ln;
-        char *delimp = delim;
+        const char *delimp = delim;
 
         /** Truncate at a full byte */
         if (outlen <= 2) {