NH QueryOver FetchMany Exception

Question

I am trying to query the parent user and then them UserRoleMappings. I have something similar to the statement below:

Dim userObj As User = session.Queryable(Of User)().Where(Function(x) x.username.Equals("someone")).FetchMany(Function(y) y.UserRoleMappings).SingleOrDefault()

When I try to run this statement, I get the following exception:

A fetch request should be a simple expression of member access; 'Convert (y.UserRoleMappings)' is instead a UnaryExpression. Parameter Name: relatedObjectSelector

I can confirm that I got this to work in the C # Test Case, but unfortunately I need it to work in VB. Can anyone help?