Calling GetVirtualInput Within SetUsageType Method Causes Error 0xC0047041 On Return

I've been trying to create an async transform component and one of the things I'd like to do is add a new output and/or columns to existing outputs for each input column that is selected.
I thought that the SetUsageType method would be a good place to do this as it will allow me to delete output's and columns based on the user's selections but, despite the fact that my code doesn't actually throw or cause errors, when I return from the SetUsageType method I'm informed that error 0xC0047041 (DTS_E_OBJECTNOTINHASHTABLE) has occurred .
I pared the method down to an absolute minimum and found that a call to GetVirtualInput is enough to push it over the edge even if it does nothing! What is wrong here?
Thanks
Charlie

Sample:
public override IDTSInputColumn90 SetUsageType(int inputID, IDTSVirtualInput90 virtualInput,
int lineageID, DTSUsageType usageType)
{

if (usageType == DTSUsageType.UT_READONLY || usageType == DTSUsageType.UT_READWRITE)
{
ComponentMetaData.InputCollection[0].GetVirtualInput();
}

return base.SetUsageType(inputID, virtualInput, lineageID, usageType);;
}