# String interpolation errors (only with logback.xml)

**URL:** <https://community.gatling.io/t/string-interpolation-errors-only-with-logback-xml/4022>\
**Category:** Gatling (Open-Source)\
**Created:** [October 31, 2017, 6:02pm UTC](https://community.gatling.io/t/string-interpolation-errors-only-with-logback-xml/4022 "2017-10-31T18:02:12Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![thms.hmm](https://avatars.discourse-cdn.com/v4/letter/t/b3f665/32.png) [@thms.hmm](https://community.gatling.io/u/thms.hmm)\
**Post date:** [October 31, 2017, 6:02pm UTC](https://community.gatling.io/t/string-interpolation-errors-only-with-logback-xml/4022/1 "2017-10-31T18:02:12Z")

</div>

Hello,

I am using Gatling 2.3 with a simple custom feeder (Map with two keys CreatedAt, Payload) and the following request body:

```auto

StringBody("""{ "created_at": "${CreatedAt}", "payload": "${Payload}" }""")

```

Running a simulation the output looks like this:

`

18:42:56.263 [ERROR] i.g.h.a.s.HttpRequestAction - ‘httpRequest-4’ failed to execute: No attribute named ‘CreatedAt’ is defined