VB.NET: how to handle runtime error "Unable to cast object of type 'Infragistics.Win.UltraWinTabControl.UltraTab' to customized type 'xxx.Tools.Windows.TabObject'

Our application is vb.net applicaiton.

we have created customized object called "TabObject".

Public Class TabObject
    Inherits Infragistics.Win.UltraWinTabControl.UltraTab
    Public Sub New()
        MyBase.New()
    End Sub  Private mbAllowEdit As Boolean = True
    Public Sub New(ByVal bPassParamter As Boolean)
        MyBase.New(bPassParamter)
    End Sub   
    Public Property AllowEdit As Boolean
        Get
            Return mbAllowEdit
        End Get
        Set(value As Boolean)
            mbAllowEdit = value
        End Set
    End Property
End Class

Then we replace

"Dim UltraTab1 As xxx.Tools.Windows.TabObject = New xxx.Tools.Windows.TabObject()"

We build solution and run the solution without problem.

Later, we open one form in design and save. the above line change to 

"Dim UltraTab1 As MenuLink.Tools.Windows.TabObject = CType(New Infragistics.Win.UltraWinTabControl.UltraTab(), xxx.Tools.Windows.TabObject)"

it builds fine and when running the application and get the following error:

"System.InvalidCastException: Unable to cast object of type 'Infragistics.Win.UltraWinTabControl.UltraTab' to type 'xxx.Tools.Windows.TabObject'."

Does anyone know how to fix this problem? Thx

JaneC