Form control checkbox linkedcell reference failing

I've devised a linked cell reference for a form control checkbox, .LinkedCell = Worksheets("sheet1").Range("D" & myCell.Row & "").Value, that will unfortunately not link to the specified cell. Although, the caption reference does work. I've placed the entire macro below:

Sub test()

    Dim myBox As CheckBox
    Dim myCell As Range

    Dim cellRange As String

    With ActiveSheet
        For Each myCell In .Range("B10:B12").Cells
            With myCell
                Set myBox = .Parent.CheckBoxes.Add(Top:=.Top, Width:=.Width, Left:=.Left, Height:=.Height)

                With myBox
                    .LinkedCell = Worksheets("sheet1").Range("D" & myCell.Row & "").Value
                    .Caption = Worksheets("sheet1").Range("B" & myCell.Row & "").Value
                    .Name = "checkbox_" & myCell.Address(0, 0)
                End With

                .NumberFormat = ";;;"
            End With

        Next myCell
    End With
End Sub

I've tried to reference the cell in this way also: .Range("sheet1!B" & myCell.Row).Value. This method fails completely.

Thanks for any suggestions.

Regards,