ok....I don't work with nChar much, but I would have thought you'd need to specify a length in your parameter.
I modified one of my simple SPs to receive the parameter as an nChar...if I didn't specify a width in the parameter list at the top of the SP, (e.g. nChar(10), it didn't work, but the minute I specified a width, it worked fine.
I don't know if that's the full answer, but give that a try.
Kevin