--- a/tmux.h
+++ b/tmux.h
@@ -537,6 +537,7 @@
 	MODEKEYCHOICE_UP,
 
 	/* Copy keys. */
+	MODEKEYCOPY_APPENDSELECTION,
 	MODEKEYCOPY_BACKTOINDENTATION,
 	MODEKEYCOPY_BOTTOMLINE,
 	MODEKEYCOPY_CANCEL,