That plots the output of your fitness function, and I find it interesting that complex results even work in that context.
I have no idea what your fitness function does, however if it is calculating the square root of a negative number or taking the logarithm of a negative number, or something similar, it would likely be best to constrain the parameters (or do whatever else may be necessary) to avoid those, to be certain that it is not generating complex results.
The algorithm may run smoothly, however it may not be doing what you want it to do.
