DragEnter doesn't work

Question

private void checkedListBox1_DragEnter(object sender, DragEventArgs e)
{
    e.Effect = DragDropEffects.Copy;
}


When I drag a file over the CheckedListBox, no "drag effect" is shown.

The Form's AllowDrop is set to true.

checkedListBox1's AllowDrop is set to true.

checkedListBox1 is contained within a groupbox.