assert_tag not working

Hi all, in a test I'm doing I'm looking for a div of class errorExplanation to know if an update passed validation, but even if the rendered html has it, as you can see below, the assert_tag fails. What am I doing wrong?

assert_tag is quite literally looking for a div tag of that class,
which your output does not contain. It does contain javascript for
inserting such a tag, but assert_tag doesn't understand that. You probably want to use assert_select_rjs (I also find the arts
plugin useful for this sort of stuff)