[Zope] Incrementing an Integer

[James Punteney]

Hello,

I'm having a problem trying to increment an integer here is the code:
<dtml-in "sqlView.expressCampaigns()">
	<dtml-call "REQUEST.set('ctn', CTN + 1)">
</dtml-in>

I have a SQL Query that returns a column called "CTN" in the database 
(MySQL) this is defined as an integer, and currently has the value of 
"20000" (the table only contains one entry right now).  I want to set the 
variable "ctn" to the CTN returned from the query plus 1.

This is the error I get:
Error Type: TypeError
Error Value: illegal argument type for built-in operation

When I view the source of the page I just see CTN: 20000, which I had 
printing out at one point but have since removed, but no error information 
or anything.

Any thoughts would be appreciated.

Thanks,
--James