Undefined index error when used with entity reference

I have a content type with an entity reference field with these parameters for the field:

Number of values: unlimited
Target type: user
Entity selection: Views - filter by an entity reference view

The error I see is:

Notice: Undefined index: #type in checkall_element_process()

and it refers to line 201 in checkall.module:
if ($element[$key]['#type'] == 'checkbox')

There's an extra $element[$key] with attributes including #autocomplete_path, #tree, and #sorted, and it is missing the #type attribute, causing the error.

I tried removing the entity reference view filter and displaying all users for this field, and I'm still getting the error.

Despite the PHP error, the check all | uncheck all boxes ARE appearing and working correctly on the form.