Error: Catchable fatal error: Object of class stdClass could not be converted to string in

I'm trying to make an html table equal to my database table, however, on the page that is in the table the following error appears:

  

Catchable fatal error: Object of class stdClass could not be converted   to string in

The part of the code responsible for the error is:

<?php
//Conexão e consulta ao Mysql
$con = mysqli_connect('localhost', 'root', '');
mysqli_select_db($con, 'sis_tam');
$qry = mysqli_query($con, "select * from clientes");

//Pegando os nomes dos campos
$num_fields = mysqli_num_fields($qry);//Obtém o número de campos do resultado

for($i = 0;$i<$num_fields; $i++){//Pega o nome dos campos
    $fields[] = mysqli_fetch_field_direct($qry, $i);
}

//Montando o cabeçalho da tabela
$table = '<table class="table table-hover table-inverse" style="margin-top:50;background-color: #37444a; color:lightgrey;"> <tr>';

for($i = 0;$i < $num_fields; $i++){
    $table .= '<th>'.$fields[$i].'</th>'; //ESSA É A LINHA DO ERRO
}

//Montando o corpo da tabela
$table .= '<tbody style="
    background-color: #86979e;
    color: #37444a;    
">';
while($r = mysqli_fetch_array($qry)){
    $table .= '<tr>';
    for($i = 0;$i < $num_fields; $i++){
        $table .= '<td>'.$r[$fields[$i]].'</td>';
    }

    // Adicionando botão de exclusão
    $table .= '<td><form action="banco/deleteC.php" method="post">'; //formulário com método post que vai para deleteF.php
    $table .= '<input type="hidden" name="ID" value="'.$r['ID'].'">';
    $table .= '<button  class="btn btn-danger">Excluir</button>'; //aqui está o seu botão
    $table .= '</form></td>';
}

//Finalizando a tabela
$table .= '</tbody></table>';

//Imprimindo a tabela
echo $table;

?>

The line being pointed to in the error is:

$table .= '<th>'.$fields[$i].'</th>';

I do not use PDO and I do not understand what might be causing the error, could anyone help me?