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fitting - FindFit is producing a list of imaginary numbers when there is non


I am trying to find the fit of an equation to some data and mathematica thinks there is a list of imaginary values. My data is all real positive number.


FindFit[Data, z*(x + shift)^2*Exp[y*Sqrt[x + shift]], {z, y, shift}, x]



FindFit::nrlnum: The function value {-122.656+0. I,-88.6481+0. I,-50.2375+0. I,-25.8042+0. I,-10.6896+0. I,<<42>>,6.49683 +0. I,5.78851 +0. I,1.22054 +0. I,<<450>>} is not a list of real numbers with dimensions {500} at {z,y,shift} = {1.40756,1.57721,-0.273062}. >>



The issue only happens when I add the shift variable.




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