ios - iOS attitude.roll returning random numbers

Question

I am using Xcode 6.1 and extracting the Pitch and Roll of the phone in the time honoured fashion:

valuePitch = degrees(attitude.pitch);
valueRoll = degrees(attitude.roll);

with

#define degrees(x) (180*x/M_PI)

The Pitch returns the correct values. But the Roll, even if the phone is vertical and not moving, will return ranges of numbers like: -168, -114, -131, 167.

Any ideas?