passing a string to a c++ dll

[GetFree]

hi i need to pass a string for output in a dll function defined like this
GenerateLicenceID(CString sBaseID, int duration, CString *sLicenceID);

and in the VB it is defined like this
Public Declare Function GenerateLicenceID Lib "C:\Final\bin\vproc.dll" Alias "?GenerateLicenceID@@YAXVCString@@HPAV1@@Z" (ByVal sBaseID As String, ByVal duration As Integer, ByRef sLicenceID As String)
but i get memory error and the program terminates