Dynamic Row source for Combo box on Userform.

hello forum,

i had been trying for some time about providing rowsource for a combo box on a userform.tried with the below code for updating combo box when there are values in the next column.

Private Sub UserForm_Initialize()
   With Sheets("Sheet1")
        With ComboBox1
           .RowSource = Sheet1.Range("rngDynamic").Address
            .ListIndex = 0
         End With
  End With
End Sub

rngdynamic : =OFFSET(Sheet1!$A15,0,0,COUNTA(Sheet1!$A:$A),1)

Where the rngdynamic implies the Named Range in the worksheet SHEET1 .my requirement is to update the combo box with the rowsource when there is data in the next column of the existing column...
Please help me where i m missing the code....