la libreria FPDF me presenta el siguiente error

Warning: FPDF::include(helveticab.php) [function.FPDF-include]: failed to open stream: No such file or directory in C:\xampp\php\PEAR\fpdf.php on line 550

Warning: FPDF::include() [function.include]: Failed opening 'helveticab.php' for inclusion (include_path='.;C:\xampp\php\pear\') in C:\xampp\php\PEAR\fpdf.php on line 550
FPDF error: Could not include font metric file

El codigo de la pagina es el siguiente:

$lusuar = $_REQUEST['cod'];
include('funciones/conect.php');
$sql = 'select * from entrenamiento where cod_ent = "'.$lusuar.'"';
$res = mysql_query($sql);
$fila = mysql_fetch_assoc($res);
$entre = $fila['entrenar'];

require('fpdf.php');
$pdf=new FPDF();
$pdf->AddPage();
$pdf->SetFont('Arial','B',16);
$query='select * from matriz_ent,trabajador where trabajador.cod_trabajador = matriz_ent.cod_trabajador and matriz_ent.estatus = "P" and cod_ent = "'.$lusuar.'" order by trabajador.cod_trabajador';
$registro = mysql_query($query,$con);
while($row=mysql_fetch_array($registro,MYSQL_NUM)) {
$pdf->Cell(40,10,$row[7],1);
$pdf->Cell(40,10,$row[4],1);
$pdf->Ln(10);
}
$pdf->Output();