How to solve betainc error in Polyfitn extension (by John D'Errico)?

When using 'just' four point for linear regression in polyfitn, I am often getting this error:

Error using betainc X must be in the interval [0,1].

Error in polyfitn (line 270) polymodel.p = betainc(polymodel.DoF./(t.^2 + polymodel.DoF),polymodel.DoF/2,1/2);

But four points should be enough to fit the plane in 3D space. And I am getting this error also when using more points.

What does this error message means? How to deal with it? Because I have never experienced this error using official polyfit in 2D space.

Thank you!