Cucumber: fill_in does not take label

In a Cucumber step, the following line will fill in the screen_name
text box:

fill_in “user[screen_name]”, :with => user.screen_name

but the following line will not

fill_in “Screen name:”, :with => user.screen_name

Here is the text of the form:

Screen name:

Is this expected behavior? I would rather use the label value than
the name value because the label value is easier to find out: I don’t
need to look at the source of the view.