The arguments to the :Field attribute are solely for the purpose of generating accessors. The purpose of the 'Type' argument is for adding type-checking code to the 'set' accessor.
When you put the 'Type' argument in, you triggered the code that generates accessors. That, in turn, complained about not finding an accessor name.
I suppose I should also make something like :Field('get' => 'name', 'type' => 'list') an error, as well, as 'Type' makes no sense without 'Set'.
Thanks for bring this to my attention. I'll work on making the POD a bit more clear.