What does this error message mean?

I’ve got an error in my app that I cannot figure out. I’ve got a
check_box_tag that I’m observing, or trying to. But Rails is throwing
an error in Firebug and not rendering the JS to the page. The only
change I’ve been able to make to get rid of the error is to simply
remove the entire observe_field line. So can anybody tell me…

What does this error message mean?

Form.Element.Serializers[method] is not a function

What is Rails looking at in the line

<%= observe_field(‘polio’, :url => {:action => ‘update_std’, :id =>
‘polio’}) %>

that it is interpreting to be a ‘method’ or a ‘function’?

TIA,
Bill