Cucumber Test Cannot find field when its there

Cucumber Test Cannot find field when its there

While I was writing cucumber test code, I get:
Unable to find field "username" (Capybara::ElementNotFound)
But I have the following on the page itself.
<%= f.text_field :username, :placeholder => "Username" %>
I've checked that it lands on the correct page using
save_and_open_page
fill_in "username", :with => "TESTUSER"
isn't the tag :username? What am I supposed to write instead?