I have some flow curves of a non-Newtonian shear-thinning polymer measured on a rheometer. Upon fitting it with the power law model (mu = K*gamma^(n-1)), the value of n I get is less than zero. Is this physically possible? Are there any examples of materials that consistently give a negative power law index? If not, what could be the reason for getting such a flow curve?

Note that analytical calculations and simulations start crashing already as n approaches zero.