Can't get roChannelStore.StoreChannelCredData to work. Always returns status 400

In a task node, I'm trying to do automatic account linking using a token. But the roChannelStore.StoreChannelCredData ALWAYS returns 400, even in the simplest scenario:

Sub Init()

    m.top.functionName = "SimplestCodeToFigureOutTheProblem"

End Sub

Sub SimplestCodeToFigureOutTheProblem()

    cstore = CreateObject("roChannelStore")
    response = cstore.StoreChannelCredData("abc123") 
    print response

End Sub

<Component: roAssociativeArray> =
{
response: ""
status: 400
}

I have to be missing something, but I can't determine it from the documentation.