Your format looks a bit off to me. I’d try it more like this:
<?php $fields = array(
'author' => '<p><label for="author"><span class="req">* </span>' . __( 'Name' ) . '</label><input id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . ' /></p>',
'email' => '<p><label for="email"><span class="req">* </span>' . __( 'Email' ) . '</label><input id="email" name="email" type="text" value="' . esc_attr( $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></p>',
'url' => '',
);
$theme_defaults = array(
'comment_field' => '<p class="comment-form-comment">' . '<label for="comment">' . __( '' ) . '</label>' . '<textarea id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea>' . '</p><!-- #form-section-comment .form-section -->',
'comment_notes_after' => '',
'title_reply' => 'Leave a Comment',
'comment_notes_before' => '',
);
comment_form( $comment_args ); ?>
Source: WordPress Codex