E-Fastrack and "Sponsor" Passport

So I was going to start doing the E-Fastrack application for the fiancee visa yesterday, but I've hit a wall so I turn to you all.

I fill in everything up to the point where the application asks about my fiancees passport information. Aquila, my fiancee, is a British citizen but the passport she did have apparently is expired and she doesn't have it. However, there's no way around the passport question on the online application form...even though information about supporting document usage states: "If your fiance(e) is a British citizen by birth but does not hold a British passport then a full (long form) British birth certificate giving parents’ names will suffice." So I'm pretty much like  . If anyone has any insight into this, well,  . Thanks!

My hubby didn't have a passport so when it asked for the number I put N/A and for the expirations date I put the present date. And this was acceptable.
 However, since your fiance has had a passport you made need to provide that number even though it has expired.