Underscores in column names

I have a underscores in most all of the column names in this database.
I've ran into a problem where Postgres doesn't like them.

SELECT * FROM "NATAB" WHERE
"NATAB"."NA_LAST_NAME" LIKE 'MITCHELL%' AND
SUBSTRING("NATAB"."NA_NAME",0,"NATAB"."NA_COLON") LIKE 'MARK%'

Produces the error : "ESCAPE string must be empty or one character"

The column "NA_COLON" is a column that holds the numeric position at
which the first name ends and the last name begins. If the column name
does not contain an underscore it works fine. Any suggestions?

- Mark