I receive the following error when creating a CHECK constraint that references another column. According to the good old Wrox SQL Server book, I'm using the correct syntax. Anyone have any ideas???

Thanks in advance!

Server: Msg 8141, Level 16, State 1, Line 1
Column CHECK constraint for column 'end_date' references another column, table 'Session'.

Here's an example of the script that I'm using:
CREATE TABLE Session (
session_key char(18) NOT NULL,
course_key char(18) NOT NULL,
site_key char(18) NOT NULL,
instructor_key char(18) NOT NULL,
start_date smalldatetime NULL,
end_date smalldatetime NULL
CHECK (end_date >= start_date)
)