diff --git a/emmental/buttons.py b/emmental/buttons.py
index 19cba81..f9e80e6 100644
--- a/emmental/buttons.py
+++ b/emmental/buttons.py
@@ -75,6 +75,7 @@ class SplitButton(Gtk.Box):
         self.emit("clicked")
 
     def activate(self, *args) -> None:
+        """Activate the primary button."""
         self._primary.activate()
 
     @GObject.Property(type=Gtk.Button, flags=GObject.ParamFlags.READABLE)