H-4 Extension until passport expiry date or H1B petition end date

Hello Everyone,

I am applying for H-4 extension for my wife. Her I-94 will expire 09/04/2017. I am on H1B and my petition is valid until Feb 2020. But her passport is expiring May 2018. So in I-539 application, should I request the extension date to her passport expiry date or my petition end date? It makes sense that passport should be valid for duration of the request but I would like to take comments from people with similar experience or experts. Appreciate in advance.