Uncaught Error: Call to a member function scaffoldSearchField() on string

gistfile1.txt

<?php

use SilverStripe\ORM\DataObject;

class TestBlock extends DataObject
{
    private static $summary_fields = [
        'getReadableState' => [
            'title' => 'State'
        ]
    ];

    /**
     * @return string
     */
    public function getReadableState()
    {
        return 'Test';
    }
}

Fix is to return an object explicitly DBText::create('Test')