Testing the content of a flash message: `The assertion was not run because of an invalid css selector`

I would like to test whether a flash message contains certain text. So I have the line:

assert_select flash[:info], "Your fee is valid till"

This however fails with the error message:

DEPRECATION WARNING: The assertion was not run because of an invalid css selector.
unexpected '#' after ''

Does anyone understand what I'm doing wrong?