diff --git a/frontends/riscos/buffer.c b/frontends/riscos/buffer.c
index c63a270db..9c8f8fe11 100644
--- a/frontends/riscos/buffer.c
+++ b/frontends/riscos/buffer.c
@@ -81,7 +81,7 @@ static os_mode mode;
  *
  * \param redraw the current WIMP redraw area to buffer
  */
-void ro_gui_buffer_open(wimp_draw *redraw)
+void ro_gui_buffer_open(const wimp_draw *redraw)
 {
 	int size;
 	int total_size;
diff --git a/frontends/riscos/buffer.h b/frontends/riscos/buffer.h
index a683c324c..7de0ecdc6 100644
--- a/frontends/riscos/buffer.h
+++ b/frontends/riscos/buffer.h
@@ -25,7 +25,7 @@
 
 #include "oslib/wimp.h"
 
-void ro_gui_buffer_open(wimp_draw *redraw);
+void ro_gui_buffer_open(const wimp_draw *redraw);
 void ro_gui_buffer_close(void);
 
 #endif