ERB syntax error

[jim-12gigs]

This (valid) Rails helper shows a syntax error, but it shouldn't.

<%= form_for(:post) do |f| %>
<% end %>

Removing the equal sign eliminates the syntax error, but also the generated code.

[jim-12gigs]

Playing around with it; apparently, this format is valid, as ERB is not trying to concat part of a do/end block.
I still don't understand why the same ERB generates the correct code in Rails, but fails to parse it in TextMate.

<%= 
  form_for(:post) do |f|
    content_tag('input')
  end 
%>