diff --git a/src/util/string.c b/src/util/string.c
index d6a213463..504fd00e6 100644
--- a/src/util/string.c
+++ b/src/util/string.c
@@ -318,7 +318,7 @@ add_crlf_to_string(struct string *string)
 }
 
 inline struct string *
-add_string_to_string(struct string *string, struct string *from)
+add_string_to_string(struct string *string, const struct string *from)
 {
 	assertm(string && from, "[add_string_to_string]");
 	if_assert_failed { return NULL; }
diff --git a/src/util/string.h b/src/util/string.h
index 748e71709..4396fdaf7 100644
--- a/src/util/string.h
+++ b/src/util/string.h
@@ -160,7 +160,7 @@ void done_string(struct string *string);
 struct string *add_to_string(struct string *string,
 			     const unsigned char *source);
 struct string *add_char_to_string(struct string *string, unsigned char character);
-struct string *add_string_to_string(struct string *to, struct string *from);
+struct string *add_string_to_string(struct string *to, const struct string *from);
 struct string *add_file_to_string(struct string *string, unsigned char *filename);
 struct string *add_crlf_to_string(struct string *string);