diff --git a/src/util/conv.c b/src/util/conv.c
index 248f52dde..cc800a16c 100644
--- a/src/util/conv.c
+++ b/src/util/conv.c
@@ -295,7 +295,7 @@ add_html_to_string(struct string *string, unsigned char *src, int len)
 
 /* TODO Optimize later --pasky */
 struct string *
-add_quoted_to_string(struct string *string, unsigned char *src, int len)
+add_quoted_to_string(struct string *string, const unsigned char *src, int len)
 {
 	for (; len; len--, src++) {
 		if (isquote(*src) || *src == '\\')
diff --git a/src/util/conv.h b/src/util/conv.h
index 494a36bba..25c499683 100644
--- a/src/util/conv.h
+++ b/src/util/conv.h
@@ -80,7 +80,7 @@ add_string_replace(struct string *string, unsigned char *src, int len,
 struct string *add_html_to_string(struct string *string, unsigned char *html, int htmllen);
 
 /* Escapes \ and " with a \ */
-struct string *add_quoted_to_string(struct string *string, unsigned char *q, int qlen);
+struct string *add_quoted_to_string(struct string *string, const unsigned char *q, int qlen);
 
 /* Adds ', |len| bytes of |src| with all single-quotes converted to '\'',
  * and ' to |string|. */