diff --git a/src/formhist/formhist.c b/src/formhist/formhist.c
index 7773edcc..d0ab7373 100644
--- a/src/formhist/formhist.c
+++ b/src/formhist/formhist.c
@@ -362,7 +362,7 @@ never_for_this_site(void *form_)
 }
 
 char *
-get_form_history_value(char *url, char *name)
+get_form_history_value(const char *url, const char *name)
 {
 	struct formhist_data *form;
 
diff --git a/src/formhist/formhist.h b/src/formhist/formhist.h
index 24662268..5f267302 100644
--- a/src/formhist/formhist.h
+++ b/src/formhist/formhist.h
@@ -28,7 +28,7 @@ struct formhist_data {
 
 /* Look up @name form of @url document in the form history. Returns the saved
  * value if present, NULL upon an error. */
-char *get_form_history_value(char *url, char *name);
+char *get_form_history_value(const char *url, const char *name);
 
 void memorize_form(struct session *ses,
 		   LIST_OF(struct submitted_value) *submit,