Can't upload APK via API: APK alias is already used by another APK?

Hi there, I'm trying to use the App Submission API to upload new APKs for my app. The first upload works fine and seems to always have a name of "null2" which I can see when viewing my upcoming version in the developer portal or via the API.

The problem is when I try to upload another APK. I get the following error:

{
    "errorCode": "error_apk_nickname_conflict",
    "errorMessage": "APK alias is already used by another APK."
}

This works fine if I upload the APK in the developer portal but not the API. How can I control the name/alias of my uploaded APKs? Where does this "null2" name come from?

I'm using the POST /{apiVersion}/applications/{appId}/edits/{editId}/apks/upload endpoint. I'm also setting a fileName header but it doesn't seem to do anything.

Any help would be greatly appreciated.

question