Using nlparci and getting the error Cannot compute confidence intervals for complex values

Hi,

I'm trying to get the confidence intervals using the following code:

options=optimset('display','iter', 'MaxIter', 500000,'TolX',1e-12,'TolFun',1e-12);
[d1,resnorm,residual,outputinfo,jacobian]=lsqnonlin('fitsTestPowerLaw',Eparams,lb,ub,options);
%fitsTestPowerLaw(Eparams)

ci=nlparci(d1,residual,'jacobian',jacobian,'alpha',0.05);

And I keep getting the error Cannot compute confidence intervals for complex values. If I look at the workspace, d1 and the residuals have no complex values, and just in case I've used the function real on both d1 and residual, but still no luck. Thanks, Charles