SQL parameter causes "Operation not applicable"

I have a strange problem.

Queries of the form

Select * from scenario
where sceCode = :sceCode

are failing with "Operation not applicable"

However I can set up the SQL like this:

Select * from scenario
where sceCode = '6'

and it works.

The parameters fail me no matter how I set their value (design time,
run-time as a constant value, run-time from another database field.)

I'm talking to a SQL Server 6.5 database.

My hair is rapidly thinning.....

Steve

p.s. please also reply to ste...@excalibur.com.au as I have a news server
that is rather aggressive in its expiry of news.