number_format hatası?

[23-Oct-2013 13:42:04 UTC] PHP Warning: number_format() expects parameter 1 to be double, string given in tag.php on line 54

hatanın sebebi nedir?

<?php

echo $price = number_format(get_post_meta( $post->ID, 'premopressFeaturedPrice', true ));

echo ' '.$currency; 

?>

echo $price = number_format(get_post_meta( $post->ID, 'premopressFeaturedPrice', true ));

bununla değiştir

echo $price = number_format(get_post_meta( $post->ID, 'premopressFeaturedPrice', 2 ));

örnek çıktı 2.85