# Invalid Interpolation Key

**URL:** <https://meta.discourse.org/t/invalid-interpolation-key/87944>\
**Category:** Product Support\
**Created:** [May 20, 2018, 7:10pm UTC](https://meta.discourse.org/t/invalid-interpolation-key/87944 "2018-05-20T19:10:39Z")\
**Posts on this page:** 1\
**Showing post:** 4

<div class="post-metadata">

**Author:** ![kimardenmiller](https://sea3.discourse-cdn.com/meta/user_avatar/meta.discourse.org/kimardenmiller/32/119631_2.png) [@kimardenmiller](https://meta.discourse.org/u/kimardenmiller)\
**Post date:** [December 4, 2019, 11:33pm UTC](https://meta.discourse.org/t/invalid-interpolation-key/87944/4 "2019-12-04T23:33:07Z")

</div>

I’m also confused on this topic of valid interpolation keys in templates. We previously customized templates such as `user_notifications.reply_by_email`, e.g. to say …

- Visit Topic or reply to this email to respond.
- Reply with a private message

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?

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