Haml 'nesting within a tag' error

"When using haml, an HTML tag is represented using %tagname, but in the case of an HTML tag with attributes, the representation is %tagname{:attribute => value}.

It turns out there must be no whitespace between %tagname and {:attribute => value}. This was the source of a lot of errors when I was trying to write haml forms, since the error message was the cryptic: Illegal Nesting: Nesting within a tag that already has content is illegal."