Inherits DataRow Class can't run new sub

Question

This is the code:
Public Class DrUser
    Inherits DataRow
....... 
Public Sub New()
             MyBase.New(Nothing)
  End Sub
   Public Sub New(ByVal rb As DataRowBuilder)
        MyBase.New(rb)
        Me.m_Table = CType(Me.Table, GenericDataSetTable)
   End Sub
..........
End Class
When start debug ,It show error: "Object reference not set to an instance of an object "  On the line "MyBase.New(Nothing)" 
How Can I do?  Please Help me, Thanks