I'm sure this is user error, so let me describe it real quick.
So, I'm in a functional test, and this line exists in the body of the response:
<div id="div_nick_name" class='nav_item'>foo</div>
I'm attempting to do this in the test:
assert_select "div#div_nick_name" "foo"
My intent in case it's not obvious is to test that the content of the div is equal to 'foo'. The first of those two lines works, the second results in: