asp.net 3.5 EnableValidator javascript problem

Question

I have the following javascript function on a page that is hosted on iis6 on win2k3 sp2 64 bit. The application is asp.net 3.5 and this is the only page in the app. This code works fine on a developer machine and on one of our server environments but fails with a null reference exception on "ValidatorEnable" on another server environment. I could not find any obvious difference between the two environments. Suggestions?

function ValidatePage(RFValidationGroup)
{
...            
for (i = 0; i < Page_Validators.length; i++) {
//This fails with function not defined
ValidatorEnable(Page_Validators[i], true);
...