diff --git a/src/util/conv.c b/src/util/conv.c
index 77cd102c2..5a43dbc65 100644
--- a/src/util/conv.c
+++ b/src/util/conv.c
@@ -272,7 +272,7 @@ add_string_replace(struct string *string, unsigned char *src, int len,
 }
 
 struct string *
-add_html_to_string(struct string *string, unsigned char *src, int len)
+add_html_to_string(struct string *string, const unsigned char *src, int len)
 {
 
 #define isalphanum(q) (isalnum(q) || (q) == '-' || (q) == '_')
diff --git a/src/util/conv.h b/src/util/conv.h
index 25c499683..86d220ccc 100644
--- a/src/util/conv.h
+++ b/src/util/conv.h
@@ -77,7 +77,7 @@ add_string_replace(struct string *string, unsigned char *src, int len,
 	add_string_replace(str, src, len, '*', '.')
 
 /* Convert reserved chars to html &#xx */
-struct string *add_html_to_string(struct string *string, unsigned char *html, int htmllen);
+struct string *add_html_to_string(struct string *string, const unsigned char *html, int htmllen);
 
 /* Escapes \ and " with a \ */
 struct string *add_quoted_to_string(struct string *string, const unsigned char *q, int qlen);