Compile error generating tye library when adding putter to and idl file and matching c++

I am getting this error when I am adding something to my idl file.

1>midl\oleaut32.dll : error MIDL2020: error generating type library : LayOut failed : Ithis(0x80029C83)

If I comment this part out it compiles oK.

   [propput] IDLAPI This([in] IThis* thisIt);



  STDMETHOD(put_Board)( IThis* thisIt)
  {
    return S_OK;
  }

Any ideas?????? I have tried so many things to make sure it all matches OK.

what does this error mean

THanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!