Invalid gamma readings can be caused by problems either with your measurement device or your monitor. To troubleshoot the problem, first try running i1Diagnostics to be sure that your device is functioning correctly.
Assuming that your device does not have any problems, be sure to clean your screen to eliminate any fingerprints or smudges. Make sure that your device is flat against the screen on all sides. Reset your monitor to factory defaults and try the calibration again.
If you continue to have trouble, try calibrating a different monitor with the same device to see if you can duplicate the problem there as well. If so, your hardware may need repair or replacement.