Unexpected issue when using attachment_fields_to_edit filter

I am developing my plugin to add a custom attachment fields. I used attachment_fields_to_edit filter (https://codex.wordpress.org/Plugin_API/Filter_Reference/attachment_fields_to_edit) to resolve my mission. However, when I go the attachment form page, it shows a field "Required fields are marked *". You can have a look at below picture.

Edit: added my soure code

function my_add_attachment_location_field( $form_fields, $post ) {
    $field_value = get_post_meta( $post-ID, 'location', true );
    $form_fields['location'] = array(
        'value' = $field_value ? $field_value : '',
        'label' = __( 'Location' ),
        'helps' = __( 'Set a location for this attachment' )
    );
    return $form_fields;
}
add_filter( 'attachment_fields_to_edit', 'my_add_attachment_location_field', 10, 2 );

function my_save_attachment_location( $attachment_id ) {
    if ( isset( $_REQUEST['attachments'][$attachment_id]['location'] ) ) {
        $location = $_REQUEST['attachments'][$attachment_id]['location'];
        update_post_meta( $attachment_id, 'location', $location );
    }
}
add_action( 'edit_attachment', 'my_save_attachment_location' );