Deleting Tablix VB .NET

Hi,

I am building a rdlc in VB 2012 and using Tablix.

There is 2 tablix.. and a criteria if GroupA=True the delete tablixB, else delete tablixA.

        If GroupA Then
            Dim tablixB As XmlNode = FindNode(XmlLayout, "descendant::ns:Tablix[@Name='DetailTabliB']/descendant::ns:TablixRowHierarchy/ns:TablixMembers")
            tablixB.RemoveAll()
        Else
            Dim tablixA As XmlNode = FindNode(XmlLayout, "descendant::ns:Tablix[@Name='DetailTabliA']/descendant::ns:TablixRowHierarchy/ns:TablixMembers")
            tablixA.RemoveAll()
        End If

but I encounter error when trying to delete one of the tablix...

Please help...