UserWarning: No contour levels were found within the data range

I am running the exact example give in this SVM example of Scikit learn without any modification.

I get the following warning.

<ipython-input-49-ba88740f796f>:14: UserWarning: No contour levels were found within the data range.
  ax.contour(XX, YY, Z, colors='k', levels=[-1, 0, 1], alpha=0.5,

Could anyone suggest how to resolve this? TIA