I made a mistake when I created one of my UIViewController objects. When I created the files, I forgot to select the Target for iPad option.
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Change it in Build Settings -> Targeted device family OR under Devices in Summary tab.
You'll find the above in your project's Targets section.
EDIT: Sorry for the misunderstanding. THIS SO post seems to address your issue.
Try this at your own risk:
First, back-up your original Xib file. Next, modify the second line from:
<archive type="com.apple.InterfaceBuilder3.CocoaTouch.XIB" version="8.00">
to
<archive type="com.apple.InterfaceBuilder3.CocoaTouch.iPad.XIB" version="8.00">
Next, open the source code of the xib, and in every ocurrence of the element:
<string key="IBDocument.TargetRuntimeIdentifier">
Replace the value IBCocoaTouchFramework with IBIPadFramework.
Good luck!