Custom font doesn't show up in app

I have downloaded Open Sans font and added all the

.tff

files into my Xcode project, and have checked the checkbox in "Target Membership" pane for each file. I have added the

UIAppFonts

key and values in Info.plist, and I am sure that the values are typo-free. Plus, the

.tff

files are added to "Copy Bundle Resources", and the custom font is showing up in Interface Builder. And then I changed the font of all the labels to Open Sans in the Interface Builder (no code).

But when I run the app in the simulator, all the labels and buttons are showing the system font in ultra small sizes, like this:

(picture)

One thing to note is that I am using Xcode 7 beta 5, and the app's Base SDK is iOS 9.

But why is there this problem? Is it a bug? Thanks!