GetElementById With Invalid ID Causes Script To Exit?

I am writing a web site that uses JavaScript to validate certain forms
and I seem to be having some trouble. The site uses PHP and for one of
the forms, depending on the situation, one of of the text inputs does
not exist - the PHP doesn't generate the HTML for it. I wanted to have
the validation script check the length of the input if it exists but I
knew that I might have trouble if I tried to check the value of the
input without first checking that the input exists so I tried using
code similar to this:

if(document.getElementById("someId")!=null &&
document.getElementById("someId").value.length<8)
{
alert("String must be at least 8 characters.");
}

However, when I try that in Firefox (that's the only browser I've tried
so far), the validation script exits, the form is submitted with
errors, and I get a message in the JavaScript console saying that the
ID that I gave was invalid. I know it's invalid! That's why I checked
for null!

I thought that getElementById() would return null if the ID was
invalid. Is there any way to check if the input exists before
validating its value?