diff --git a/src/document/options.c b/src/document/options.c
index 752eaa9b1..f11c60220 100644
--- a/src/document/options.c
+++ b/src/document/options.c
@@ -144,7 +144,7 @@ done_document_options(struct document_options *options)
 }
 
 void
-toggle_document_option(struct session *ses, char *option_name)
+toggle_document_option(struct session *ses, const char *option_name)
 {
 	struct option *option;
 
diff --git a/src/document/options.h b/src/document/options.h
index f98c790dc..84829a226 100644
--- a/src/document/options.h
+++ b/src/document/options.h
@@ -191,7 +191,7 @@ int compare_opt(struct document_options *o1, struct document_options *o2);
 /** Increments the numeric value of the option identified by @a option_name,
  * resetting it to the minimum value when it is already at the maximum value,
  * and redraws the document. */
-void toggle_document_option(struct session *ses, char *option_name);
+void toggle_document_option(struct session *ses, const char *option_name);
 
 #ifdef __cplusplus
 }