Hi Alex,
Sorry you are having problems with the tool! What version of the software are you using? (ArcGIS 10.2 or ?). You should not see that error unless you truly have all the same values so that's odd. Is there anyway that you can send me your data so that I can try to reproduce the problem (you can remove all fields except the one that is giving problems)? If so, please email me directly at LScott@esri.com.
Again, sorry this isn't working for you!
Lauren