Simple question: promptusertext method

I'm a novice in IDL, so please bear with me. I want a procedure to
display a prompt with a text field. From what I understand,
promptusertext is the method I should use. This said, I can't get it
to work properly. The prompt never displays and just registers the
user's entry as 0. Here's my code:

PRO ...

....

obj = Obj_New("IDLitIMessaging")

result = obj->promptusertext('...', answer, title='...')

....

END

What am I doing wrong?