NullReferenceException thrown even though checking for Nothing

NullReferenceException thrown even though checking for Nothing

I have some rather straightforward code for a Compare Function
Public Overridable Function Comparer(thisValue As Object, otherValue
As Object) As Integer
Try
If thisValue Is Nothing Then
If otherValue Is Nothing Then
Return 0
Else
Return -1
End If
Else
If otherValue Is Nothing Then
Return 1
Else
Return thisValue.ToString.CompareTo(otherValue.ToString)
End If
End If
Catch ex As Exception
Return 0
End Try
End Function
The reason for the try-catch block is: I get a NullReferenceException at
the actual comparision line if thisValue is Nothing. The debugger shows me
that thisValue is "Nothing" but lands in the ELSE branch anyway.
Can anyone tell me why?