500 response on retrieval of ref through REST

This issue affects Bitbucket Pipelines.

When making following REST request to retrieve a non existing branch ('non-existing':

https://api.bitbucket.org/2.0/repositories/jeroendr/meat-demo2/refs/branches/non-existing

With body:

{
error: {
message: "u'non-existing'",
id: "3a2ac67ef1f04b4797c5a54c133da624"
}
}

the Bitbucket REST API returns a 500 whereas I was expecting a 404 + the message should not be prepended with the u'.