diff --git a/src/document/forms.c b/src/document/forms.c
index b97bd9f23..7303fb6f6 100644
--- a/src/document/forms.c
+++ b/src/document/forms.c
@@ -42,7 +42,7 @@ static struct form_type_name form_type2name[] = {
 #define FORM_TYPE_COUNT (sizeof(form_type2name)/sizeof(struct form_type_name))
 
 enum form_type
-str2form_type(char *s)
+str2form_type(const char *s)
 {
 	int n;
 
diff --git a/src/document/forms.h b/src/document/forms.h
index 1c7475e47..2df721f1a 100644
--- a/src/document/forms.h
+++ b/src/document/forms.h
@@ -128,7 +128,7 @@ struct el_form_control {
 };
 
 /* Numerical form type <-> form type name */
-enum form_type str2form_type(char *s);
+enum form_type str2form_type(const char *s);
 char *form_type2str(enum form_type num);
 
 struct form *init_form(void);