PHP- Trying to get property of non-object

I'm trying to do a query on mysql in php, but I get an error on line 131, which is:

$num = $res->num_rows;

The error is:

  

Trying to get property of non-object.

I know that the error comes from the query but I can not find it. Could you help me please?

This is the code:

function Enlazar($conexion,$NumAlbaran,$cont,$empCont,$fecha2){
    $conseguido = true;
    //buscamos su entrega
    //mala sentencia sql
    $ocup = 'ocupado';
    $ent = 'Entrega';
    $res = mysqli_query($conexion,"SELECT albaran.NumAlbaran,albaran.fecha,albaran.fecha_pagado FROM albaran,contenedor,solicita   WHERE contenedor.estado = '".$ocup."' and  numCont = '".$cont."' and solicita.empresa = '".$empCont."' and albaran.tipo='".$ent."'  ");
    $num = $res->num_rows;
    echo $num;
    if($num == 1 ){
        $a_res =  mysqli_fetch_assoc($res);
        $intervalo = CalcularIntervalo2($a_res['fecha'],$a_res['fecha_pagado'],$fecha2);
        //vinculamos la entrega con su recogida/residuo
        $aux = $a_res['NumAlbaran'];
        $res2 = mysqli_query($conexion,"INSERT INTO enlace (NEntrega,NRecogida,dias) VALUES ($aux,'$NumAlbaran','$intevalo') ");

        if(!$res2){
            $conseguido = false;
        }

    }
    else{
        $conseguido = false;
    }
    return $conseguido;
}

The albaran table has the fields: NumAlbaran, nomEmp, location, date, type and date_paid.

The table you request, with which you link the delivery notes with the containers, has the fields: numAlbaran, numCont, company.

The container table has: numCont, company, size, state.