Uncomprehensible error message

[jwezel]

I've got an error message like this on an optimization run:

Trial 6 failed because of the following error: The number of the values 10 did not match the number of the objectives 1.

I can't make sense of this and searching for it yielded nothing.

Can somebody explain?

[nzw0301]

Could you share the minimal reproducible code with us?

[jwezel]

Mistake: I unknowingly returned a list of values from the objective function instead of only one