error during fitting a model

I am fitting a function using "fittype" and "fit" commands. But I am getting following error again and again. Sometimes it goes away. But rarely. Please help me.

Error using fit>iFit (line 340) Complex value computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients.

Error in fit (line 108) [fitobj, goodness, output, convmsg] = iFit( xdatain, ydatain, fittypeobj, ...

Error in test2 (line 10) f=fit(x,y,ft)

[Torsten]

Show us your model function.

Best wishes

Torsten.