在POST请求中使用DB :: raw

So I have a query that is selecting a table. That is all. This query runs in an AJAX POST request in one of my controllers. Here's the query:

$myTable = \DB::table('signature')
    ->where('email_list', 'LIKE', '%'.$request['email'].'%')
    ->select([
        'signature',
        'version',
        'type',
        DB::raw("CONCAT(signature.size,'',signature.size_unit) AS sizea")
    ])
    ->get();
echo json_encode($myTable, JSON_PRETTY_PRINT);

Whenever I make the POST request, I get a error 500 Internal Server Error. If I remove/comment out the DB::raw("CONCAT(signature.size,'',signature.size_unit) AS sizea") part, the POST request goes through and everything is fine.

So there's probably something wrong with the DB::raw part, right? You'd think so, but if I use this query on the page I'm executing it on and var_dump it, the statement works, even with the DB::raw part.

Can anyone shed some light on what's going on?

EDIT: Error logs from Console:

POST                                                 jquery.min.js:4  
http://xxx.xx.xx.xxx:8000/lab/misc/signature/get_list 500 (Internal Server Error)
    send @ jquery.min.js:4
    ajax @ jquery.min.js:4
    (anonymous function) @ signature_hunter:345
    dispatch @ jquery.min.js:3
    q.handle @ jquery.min.js:3
    ListPicker._handleMouseUp @ about:blank:542