surrogateopt iterative display error

While using surrogateopt I recieve the following error only when I enable the display of iterative output messages ('Display','iter' set using optimoptions):

"Error using fprintf

Function is not defined for 'struct' inputs."

My optimization problem has non-linear constraints so I am using a function that generates a struct with a .fval field. It seems like there could be an imcompatibility between that and the output text function, but I want to see if there is something I am not doing correctly.

[Alan Weiss]

What software version are you using? Nonlinear constraint support was introduced in R2020a.

Alan Weiss

MATLAB mathematical toolbox documentation

[Alan Buehne]

2019b. I guess I need to upgrade; thank you.