I am looking at the habitat suitability of water voles for my research project.  Using the program SPSS (v21) I have computed a series of stepwise logistic regression models to predict their presence and absence in the habitat that I am looking at. They have successfully produced some results but when I try to plot a ROC curve, the following warning message comes up:

Warnings
Either or both of the positive and negative actual state groups have no valid observations. Estimation cannot be performed, and the ROC curve will not be displayed.

I have put the predicted probability values in the 'test variable' box and the variable that was included in the habitat suitability model in the 'state variable' box.  Has anyone had this warning come up before and know how to get round it?

Thank you very much.

Best wishes,