Facebook Graph API returns numbers instead of email addresses

I use Facebook Connect along with the Facebook API to get custom email addresses when registering on my site. This works perfectly in 99% of cases, but sometimes when I request the Graph API for user data after they give permission to my site, including email permission, Facebook returns a large amount (e.g. 14036774009) as the email address mail man.

So far, the numbers are always different and always contain 11 digits, and all other user data from the Graph API is valid. I could never reproduce this issue with a Facebook account that I control.

In a way, a large number reminds me of random proxy email addresses that Facebook generates for people who prefer to provide third-party applications with a forwarding address instead of their primary address (proxy addresses look something like this: apps + 148742679521093.617890126.8a2b26037e1ccd06bb81aaec5925f4c7 @ proxymail.book.com)

Can someone explain this behavior or a way to fix it (and always get valid email addresses)?