Entity Reference Autocomplete module: hook_entity_info definition?
I'm the module maintainer. Thanks for using the module, hope you're finding it useful. You need to add a 'label' key to your entity definition, like:

'entity keys' =&gt; array(
  'id' =&gt; 'grid',
  'label' =&gt; 'table-column-that-contains-the-human-name-of-entity',
),


With that, and after flushing caches, etc, your form should work fine. The reason is that entityreference_autocomplete assumes that (as a best practice), every entity would have, apart of the ID, which will be normally just a number, some friendly way to identify it through a label. Therefore, you need that on your custom entity.

However, I get that not everyone might want / need that 'label' key or a column for the label in their schema, for whatever reason, and I've added support to the module to work without that. That bit is on the dev version on the module, and will be rolled out in the next stable version. So it's up to you, to define a column for the 'label' key in your schema, or download the dev version, or wait a  couple days for the stable release that supports entities without the 'label' key.

Cheers!