Heroku push: gzip stdin not in gzip format
===

When I try to push an update to Heroku in one of my PHP apps I get the following problem:

> Counting objects: 25, done.<br />
> Delta compression using up to 4 threads.<br />
> Compressing objects: 100% (12/12), done.<br />
> Writing objects: 100% (13/13), 1.20 KiB, done.<br />
> Total 13 (delta 10), reused 0 (delta 0)<br />
> <br />
> -----> Heroku receiving push<br />
> -----> Fetching custom buildpack... done<br />
> -----> PHP app detected<br />
> -----> Run Sitebase buildpack<br />
> -----> Bundling Apache version 2.2.22<br />
> <br />
> gzip: stdin: not in gzip format<br />
> tar: Child returned status 1<br />
> tar: Exiting with failure status due to previous errors<br />
>  !     Heroku push rejected, failed to compile Php app<br />
> <br />
> To git@heroku.com:x<br />
>  ! [remote rejected] feature-removeapi -> master (pre-receive hook declined)<br />
> error: failed to push some refs to 'git@heroku.com:x'<br />

Never had this problem before so I totally don't have a clue what the problem can be.
Is it possible that this is a bug on Heroku's side?

If I look in the Heroku logs I also see the following line:

> Slug compilation failed: failed to compile Php app

All help is welcome.