If an invalid kind of dependency is passed into mg.Deps, then it's pretty hard to nail down what has happened, as the error message does not provide any location where that happened:

Error: Invalid type for dependent function: func(string, string). Dependencies must be func(), func() error, func(context.Context), func(context.Context) error, or the same method on an mg.Namespace.

It would be nice to be able to point to at least mg.Deps invocation, or, better, to the argument that caused the problem.