IOS - Parse.com - Current User nil after kill app on ios 7

I coding a ios app with parse SDK on IOS. i check user logged in by

[PFUser currentUser] != nil

I check it when the app is killed by user, and open again. It worked fine on ios 8. But when i check on ios 7.1, currentUser is nil. Look like it logged out automatic or currentUser did not save.

Oh, I using parse SDK 1.7.5. A new version is 1.8.0, maybe i should update it?

Sorry for my bad english.

via Chebli Mohamed