Undefined index: HTTP_REFERER when calling 'callAuthenticatedWs'

[keessonnema]

Mantisbt version: 2.19.0 (latest stable)
MantisApiBundle version: Latest

When executing the following callback function I get an error:
$mantisApi->callAuthenticatedWs('mc_issue_note_add', ['issue_id' => <an-issue-id>, 'note' => $note]);

Error:

+"faultstring": """
Error Type: SYSTEM NOTICE,\n
Error Description: Undefined index: HTTP_REFERER
"""
+"faultcode": "SOAP-ENV:Server"

Full log: (removed private data)
https://hastebin.com/serelirici.cs

Could this be a MantisApiBundle problem? If not, I'll look further.