Template Interpolation Key Usage

Related to this post, we previously customized templates such as user_notifications.reply_by_email, e.g. to say …

The second item has the link:

%{base_url}/new-message?username=%{username}&title=Your%20post%20%{topic_title_url_encoded}&body=Regarding%20your%20post%20in%20%{base_url}%{url}

This has been in use for some time and works perfectly. Today we need to change the text in user_notifications.reply_by_email, but on saving get the error:

The following interpolation key(s) are invalid: "username, topic_title_url_encoded"

Why the limitation now, and how can we get around it?