Curve fitting tool complex value computed by model function

I am trying to use the Matlab Curve Fitting tool to fit a dataset to the Wiebe Function, x=b*(1-exp(-2.3026*((x-start)/duration)^(m+1))). Where x is the output variable, y is the input variable and b, start, and duration are the model fitting parameters. When I manually set m to any whole number I get a solution, however, if it is decimal or set to a parameter that the tool can vary I get the following error. "Complex value computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients."

I have also constrained the bound on 'm' to 1.1 to 1.2 and I still get the same error.

Any help resolving this issue would be greatly appreciated!

[Torsten]

Hint:

x^a becomes complex if "x" is negative and "a" is not an integer.

Best wishes

Torsten.