vb.net Automation Issues - COM Interop

I've been having a few issues recently in creating a vb.net class
library (using vs.net 2003), registering it for com interop and then
invoking its functions from an object in VB6.

The error I am receiving is happening in VB6 when I go to compile the
project:

Compile Error:
Function or Interface marked as restricted, or the function uses an
Automation type not supported in Visual Basic


Here is my vb6 code, which is run from a button on a test form:

Private Sub cmdInvoke_Click()
On Error GoTo err_handler

Dim obj As CommsClass.ClientInterface
Set obj = New CommsClass.ClientInterface

obj.Invoke

Exit Sub

err_handler:
MsgBox Err & " - " & Error

End Sub


And here is my vb.net CommsClass code. This is the ONLY code I have in
the project, and the project is registered for COM Interop.

Public Class ClientInterface
Public Function Invoke() As String
MsgBox("Hi")
End Function
End Class

Why would somthing so simply cause such an awkward error?!

Thanks

.