symfit: Too many parameters?

I’m sorry if this is a stupid question, but I’m getting an error:

  File "/anaconda3/lib/python3.6/collections/__init__.py", line 434, in namedtuple
    exec(class_definition, namespace)
  File "<string>", line 12
SyntaxError: more than 255 arguments

My model looks like this: model_dict={ y1:jim_model(x1,norm11,nthr11,holes_k11,holes_tau11,interface_k11,interface_tau), y2:jim_model(x1,norm12,nthr12,holes_k12,holes_tau12,interface_k12,interface_tau) } fit = Fit(model_dict,x1=x1_data,y1=y1_data,y2=y2_data) fit_results=fit.execute()

where jim_model is a function I defined earlier. The fit works well with just one of the data sets but gives me the above error with two datasets simultaneously. Is this a case of too many parameters at once and might there be a workaround?