pass in null value for boolean data type in VB

What is the syntax to pass in null value for boolean data type in VB in the stored procedure?
This is what I tried and it doesn't work.
.Parameters.Append .CreateParameter("@.disposition", adBoolean, adParamInput, 1, vbNull)
Thank you.I'm no vb programmer (at least that's what I tell people)

How about:

.Parameters.Append .CreateParameter("@.disposition", adBoolean, adParamInput, 1, "Null")

??