Very small estimated errors

Hi!

At the moment I am using the Global Fit Package of Igor Pro 8.04 to fit various data sets with two different user-defined functions:
f(x) = B_0*exp(-C/(F*(x+1))-kS*F*x)
f(x) = B_0*exp(-C/(F*(x+1))-kS*F*x)*exp(-C/(F*(x+1)))*(pODis+pOSurf*(exp(C/(F*(x+1)))-1)),
where only F is fixed.
In general the fits are fitting my data quite well, but the given estimated errors, especially in case of kS, are unphysically small. Does someone have an idea where this could stem from and how to deal with it?

Thank you very much already in advance!