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Why does my DLL work in VB6 but not .NET?

I have a dll created using VB6. It works fine when I call it in VB6. When I call the same in a .NET application, it gives me System.InvalidCastException:QueryInterface for interface Projectname.ClassName failed. I have registered the dll in all the boxes. Everywhere it works fine except with .NET.
I can give some things to look for, but without seeing the code, I'm at a loss with this one. Since there is a casting exception being thrown, is the object typed correctly? The QueryInterface failure is interesting also. Does this object have variants as method parameters or does it return a variant? Is that where the type casting exception is occurring?

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