how to set class attribute with form_tag in rails

I have the following line of haml:

=form_tag :action => 'create', :controller => 'comments', :class => 'comment_form' do

But the html that gets output is:

<form accept-charset="UTF-8" action="/comments?class=comment_form" method="post"></form>

I want to set the class. How do I do this?

<-- Update -->

With this:

=form_tag ({ :action => 'create', :controller => 'comments' }, { :class => 'comment_form' }) do

I get this error:

 syntax error, unexpected ',', expecting ')'
...', :controller => 'comments' }, { :class => 'comment_form' }...

<-- Second Update -->

The problem above is the space between 'form_tag' and '(' @woahdae's answer is correct