I have an array of valid US state codes (StCodes) and an array of user supplied state codes in a column in table RT.
The following correctly returns an array of positions of valid codes and #N/A for invalid codes. (valid codes are like AZ, CA, etc)

                  MATCH(RT[state],StCodes,0)

I was hoping to get an array of the positions of invalid codes by wrapping the previous in another MATCH

                MATCH(NA(),MATCH(RT[state],StCodes,0),0)

but that returns #N/A

I'm looking for a solution and an explanation as to what I am doing wrong.

(Note that in the above I used square brackets to show a column name in the RT references, but I could not find a way for them to display in the post.
If I edit the post, the brackets reappear RT!state!)

Thank you.

-robert