autocomplete for entityreference fields?

i have a node type with an entityreference field. it was using a select list, but with the related data getting larger than expected, i have changed the widget type for the node form to autocomplete. when i navigate to the form, i start typing into the field, and no matter what i type, i get the entire set of related entities displayed in the autocomplete list.

on another node type, i have a termreference field with the widget type set to "autocomplete term widget (tagging)", and that one behaves exactly as i would expect.

have i missed something about autocomplete?

thanks for any ideas.