Complex stansardized Residuals

Dear all,

I am doing some nonlinear regression and use mathematica to calculate the associated FitResiduals and StudentizedResiduals.
I am a bit confused right now because one studentized residual come out as a complex number...

According to the definition:
StRes=FitRes/[Estimated Variance*(1-Hatmatrix Diagonal number)]^0.5.

How can Estimated Variance*(1-Hatmatrix Diagonal number) be negative? Clearly there is something I do not understand about how mathematica exactly computes this...

Any suggestions?

Thanks in advance,