error message when trying to fit model parameters to data-set

Can you please tell me what to do when this message appears:

Fit could not be computed due to error:

Complex value computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients.

I already tightened the parameter boundaries to the max, where I still expect reliable data. Could there be another reason why I can not get the fitting done? Granted, my function is a exotic one, derived from an equivalent circuit model.

Thank you for your help

[Star Strider]

Are you fitting real (time-domain) or complex (frequency-domain) data? Are you using nlinfit, lsqcurvefit, or the Curve Fitting Toolbox?

[RAJESH AGARWAL]

Yes