[conartist6]

Yeah I would emit NaN and just hope the receiver handles it.

What's the point of lying about the data?

The format offers you no data type that would not be an outright lie when applied to this data, so you may as well not lie and break the format

[sevensor]

Your other option is to comply with the spec, emit a string, and expect the receiver to deal with that at validation time rather than parse time.