document.form[0] is Null or Not an Object error when hiding a text field in ASP.Net

I am hiding a TextBoxin my aspx page like this:

myField.visible=false;

Now I have a DropDown as well which tries to access the TextBox开发者_如何学编程 on IndexChange. The problem is, it cant access the hiffen TextBox and I am getting document.form[0] is Null or Not an Object. How can I solve that? Is the some check for that in JavaScript?

Thanks :-)

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If you are trying to access the first form in the document via its numerical index (hint: You shouldn't be, the id attribute is very handy, then you are looking for the first in a group of forms, not the first in a group of form.

document.forms[0] // It is plural