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?

UPDATE: I have tried to amend the situation by inserting yet another check for Nothing. In my scenario this boils down to a few hundred Exceptions and the execution speed is bearable. Don't want to imagine someone trying to sort an empty column though.

https://i.sstatic.net/8dnXD.png

How is this possible? Is there another "LEVEL" of Nothingness that i don't know about. Do i need to check the type of thisValue and otherValue?

The Function is never overridden. I have tried removing the "Overridable" to no effect.

Perhaps it isn't that thisValue is Nothing, it is the fact that .ToString() is returning Nothing? Try this code to test it out:

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

                Dim thisValueStr As String = thisValue.ToString()
                Dim otherValueStr As String = otherValue.ToString()

                'HERE, CHECK THE TWO STRINGS FOR NULL!!!

                Return thisValueStr .CompareTo(otherValueStr )

            End If
        End If
    Catch ex As Exception
        Return 0
    End Try
End Function

If this is the case, double-check the implementation of ToString() in the object being passed (assuming it is a custom type).