Illegal offset type in isset or empty Laravel

i use following query to get Project Id and Id of Particular Table in Laravel.

$projectlist=Projects::Where('customer',$request->input('client_name'))->pluck(DB::raw("CONCAT('project_prefix','project_nos') AS projectid"),"id");

I got following Error.

message Illegal offset type in isset or empty
exception   ErrorException
file    /var/www/msstone/vendor/laravel/framework/src/Illuminate/Support/Str.php

via Chebli Mohamed