While trying out conversion of a raw file from a Thermo Fisher Scientific VeriSpray PaperSpray instrument, I noticed the exception in the title. I would have submitted a PR, but I don't know where the accession value comes from.

IonizationModeType.PaperSprayIonization, new CVParamType
{
    accession = "MS:???????",
    name = "paper spray ionization",
    cvRef = "MS",
    value = ""
}