I/O Adapter question

I use the I/O Adapter in the Enterprise Integrator. I want to Poll a file (File Poll), parse it’s content (Fixed Length Input) and map it to a matching Document Type that will be later used by a database adapter to save the data into a table.
When I get to the step of mapping the parsed sequenced hierarchical output to the Document Type I create I get a ‘illegal mapping’ error. The Document Type matches the parsed record exactly. What am I doing wrong?