My formula results in "NaN". What does that mean?

[B S]

I am working on form that will be emailed once completed. The user will
click on a button that will email it to a specific account. I want the
formula to automatically pull two fields and place them in the subject line
of the email along with a short text. I type in the formula and verify it
and no errors come back but, when I send it the subject line comes out with
"NaN". How can I fix this? Here is the formula I am trying to use:
string("TEC Request for - ") + string(AreaRegionCOSA + ProjectActivity)

 

[frebou]

NaN = Not a number.

He is probably trying to "sum up" the string with the number. Ensure your
using right concatenation method and that everything that you "+" are
number...

 

[Art]

I always use

concat("TEC Request for - ", AreaRegionCOSA, " ", ProjectActivity) and it
always work fine.