isValid throws 403 error

Hello,

I am attempting to use this isValid code:

<cfparam name="myVar" default="">

<cfif (myVar NEQ "") AND NOT (isValid("string",myVar))>

          Failed<p>

<cfelse>

          passed<p>

</cfif>

if i call this page as myPage.cfm?myVar=5, the page works. if i do  myPage.cfm?myVar=5555, i get a 403 error. is this a bug?