Illegal string offset trying to override WordPress comment form fields

I have no idea what I’m doing wrong here. My code looks exactly like this guy’s (method 2, toward the bottom), but it’s not working. Here’s my code:

/** Change comment form title **/
function change_comment_fields($fields) {
    if (isset($fields)) {
        $fields["title_reply"] = "Leave a Comment";
        var_dump($fields);
    }
    return $fields;
}
add_filter( 'comment_form_default_fields', 'change_comment_fields' );
add_filter( 'comment_form_field_comment', 'change_comment_fields' );

Which var_dumps:

array(4) {
  ["author"]=>
     string(207) "<p class="comment-form-author"><label for="author">Name <span class="required">*</span></label> <input id="author" name="author" type="text" value="" size="30" aria-required='true' required='required' /></p>"
  ["email"]=>
     string(235) "<p class="comment-form-email"><label for="email">Email <span class="required">*</span></label> <input id="email" name="email" type="text" value="" size="30" aria-describedby="email-notes" aria-required='true' required='required' /></p>"
  ["url"]=>
     string(125) "<p class="comment-form-url"><label for="url">Website</label> <input id="url" name="url" type="text" value="" size="30" /></p>"
  ["title_reply"]=>
     string(15) "Leave a Comment"
}

So it seems as though the array is being changed properly. “Leave a Reply” doesn’t change, but if I set ‘url’ instead of ‘title_field’, the URL box will change to “Leave a Comment”.

Nevertheless, PHP still spits out this:

Warning: Illegal string offset 'title_reply' in /home/<name>/public_html/<site>/wp-content/themes/baskerville-child/functions.php on line 14
string(113) "Lp class="comment-form-comment">

(Where line 14 corresponds to the where I’m setting $fields[“title_reply”].)

Any ideas? (You can view a sample page here.)