This is because the selectors are actually for the bones, not the geometry. The work-around is to insert a target helper object and make it a child of the eye. You can also make the light a child of the eye as well so it follows the head if you reposed the characters. The final step is to use a Point-At behavior for the light and select the target helper object for the eye. Just adjust the spread of the to remain on the eye.

Mark