NodeJS: runInNewContext and instanceof

I have a script which does something like this:

var context = {}
vm.runInNewContext("var someFunc = function() {}", context);
console.log(typeof context.someFunc); //function
console.log(context.someFunc instanceof Function); //false

I do understand why the 4th line returns false: inside the new context there is a new Function object, which is not equal to the Function object in the outer context. Therefore, context.someFunc is not an instance of that outer Function object.

However, the context.someFunc function is used by a third-party library which uses instanceof Function. Since context.someFunc is a function, but not an instance of the Function in that context, the third party won't treat it as a function while it should and therefore it crashes. I tried using the following context:

var context = {
    "Function" : Function
}

but that didn't resolve my problem either.

Perhaps using var someFunc = new Function(arg, body) will work (haven't tested it), however I do not have full control of the code passed to vm.runInNewContext and therefore I can't use that solution either.

How can I let context.someFunc instanceof Function return true outside of the context?