Saving to Cassette on Casio Z-1GR?

[toml_12953]

I have a Casio Z-1GR and the FA-6 interface unit. What is the command to save a BASIC program to cassette? I tried SAVE "filename" but I get an NR error (device not ready) I tried just SAVE alone but I get a BF error (improper filename)

There shouldn't be any feedback from a cassette interface while trying to write so I don't think the NR error refers to it which means there must be another command to use the cassette. I don't have the original Japanese manual, just a partial English translation which omits any mention of saving to cassette.