PHP Laravel Error: Object of class stdClass could not be converted to string

I am developing a feature for an app using Laravel, and I am coming across this weird error that I can't figure out. I have the following code as a helper function to my controller, which was working just fine before I added a call to the following function:

protected function f($p){}

    $cIds = $cs->select('cs.id')->get();

    $cs = DB::table('cs')
            ->select('cs.id')
            ->join('pucs', 'cs.id', '=', 'pucs.c_id')
            ->where('pucs.p_id', '=', (string)$p->id)
            ->whereIn('cs.id', $cIds)->lists('cs.id');
    return $cs;
}

where pucs has a foreign key to cs and a column called p_id, and I only want to return rows where p_id = $p->id. $p is the result of a previous query, and $p->id is an int (I think?). When I open the page on my website, I get the following error:

Missing argument 1 for Illuminate\Database\Query\Builder::lists()

Does anyone have any insight as to what might be causing this problem? My initial thought was that $p->id was an int and that was causing the problem, so I cast it to a string using the (string) operator, as well as trying strval($p->id). I'm out of ideas.

via AWeston