How do I get vim to show trailing whitespace, but use blank spaces for tabs?

Question

I want trailing whitespace to show as ., and tabs to show as blank spaces instead of ^I. I thought the solution would be easy:

set listchars=tab:  ,trail:.
set list

but that doesn't quite work:

Error detected while processing .vimrc:
line 11:
E474: Invalid argument: listchars=tab:

I suppose I have to escape or encode the whitespace somehow, but how?

Answer 1

 set listchars=tab:\ \ ,trail:.