Haml: how to generate dynamic id

I've got following code in erb which works fine: ``` <div id='flash_<%= flash.keys.first.to_s %>'> <%=h flash[flash.keys.first] %> </div> ``` I want to convert it into haml: ``` #flash_#{flash.keys.first.to_s} =h flash[flash.keys.first] ``` But I receive error: ``` Illegal element: classes and ids must have values. ``` Which is strange as there IS value, 'flash_' part is always present, I get that error even when I do: ``` #flash_#{nil.object_id} ``` Apparently something wrong is with my Ruby interpolation in haml, but I can't get it right. According to documentation http://haml.info/docs/yardoc/file.REFERENCE.html#ruby_interpolation_ #{} is used to interpolate Ruby and it works in such case: ``` #flash_ #{flash.keys.first.to_s} ``` but that's not what I want. To sum up, I want to get the following output: ``` <div id="flash_foo"> blahblah </div> ``` but it can be also: ``` <div id="flash_"></div> ``` How to obtain that with haml?