gteElementById for animate?

Hi...

I put this line in my svg doc
<animate id="migrationAnimateX" attributeName="x2" from="1" to="80" 
dur="2s"/>
Then I tried Javascript:

doc.getElementById("migrationAnimateX").setAttribute('to', x2);

But I get null from the getElementById?  Just before this code I used 
getElementById() to find a line element. Any idea why I can't get the 
animate element?

Thanks,
jjb