fitnlm problem- the SE is very small

Hello all, I am using 'fitnlm' to fit a non-linear model, however, the SE in mdl.Coeffficients is very small, in the order of e-16~e-30.

The model for sure cannot fit perfectly to the data, if I feed the data into the curve fitting toolbox using the same initial value, I can get reasonable boundary for the model coefficients.

General model:

f(x) = 1-a*x^b-c*x^d

Coefficients (with 95% confidence bounds):

a = 0.002713 (0.001471, 0.003956)

b = 0.6173 (0.5348, 0.6998)

c = 9.554e-20 (-6.96e-19, 8.871e-19)

d = 6.375 (5.121, 7.629)

Their goodness of fittting are both good.

Did anyone meet similar problems? Thank you.