How can I call a DB2 stored procedure with OUT parameters from SQuirreL SQL?

I really like SQuirreL SQL as a SQL query tool, but I've never been able to get it to call stored procedures in our AS/400 DB2 database. I always get the error "The number of parameter values set or registered does not match the number of parameters." I've double-checked the number of params and had no luck. This is the syntax I've tried for a procedure that takes one IN and one OUT:

call SOMESPROC(12345, ?);