"Invalid procedure call or argument" VBScript calling C# COM method

I have a VBScript app calling COM-registered C# objects. I am able to pass in a COM object to开发者_如何学编程 a COM call, as well as receive either a primitive or a COM object back, but I can't do both at the same time! If I try retrieving any value back from the call while also passing in a COM object, I get the exception "Invalid procedure call or argument"

Dim foo
Set foo = Server.CreateObject("Foo")
foo.SetProperty(1)

Dim bar
Set bar = Server.CreateObject("Bar")
Dim return
Set return = bar.Do(foo)

If that last line is simply bar.Do(foo) it works fine.

Also, whether it is

Set return = bar.Do(foo)

or

return = bar.Do(foo)

causes the same error in this case.

My COM classes are classes with only methods exposed, and implementing an interface. I'm getting this error by dealing with only ints, longs, and Strings.

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I'm a bit rusty in this area but if your method is returning an int or string shouldn't your code then read:

return = bar.Do(foo)

instead of

Set return = bar.Do(foo)

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"return = bar.Do(foo)" should work, as long as Bar.Do is actually returning something. How is Bar.Do defined?