javascript error on content page

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.NET2.0

Javascript

.NET

, +

hello all,

i am having a problem.When i was working in an application without master page and content page, javascript was working finely.But later i needed to use mastr paga in my project.After using master page, javascripts is giving error on call of the function.
error is like below.

"Microsoft JScript runtime error: 'document.getElementById(...)' is null or not an object"

how should i put the javascript in content page.

please help as soon as possible.

thanking you
//
function enable()
{
document.getElementById('<%= txtID.ClientID%>').disabled=true;