title: Ensure that assigning RowSource will not fail
body: <p>I have 2 ComboBoxes which I populate using <code>RowSource</code>.<br>
  Combobox2 <code>RowSource</code> change based on the value of Combobox1.<br></p>
  
  <p>I have this code:</p>
  
  <pre><code>If Combobox1.Value = "Item1" Then Combobox2.RowSource = "Sheet1!Item1"
  </code></pre>
  
  <p><code>Sheet1</code> is the name of the sheet, <code>Item1</code> is a named range.</p>
  
  <p><strong>Problem:</strong><bR>
  This works fine if the only workbook open is the one containing this code.<br>
  However, if another workbook is open, it fails to reference the <code>RowSource</code> correctly.<br>
  How can I improve this?<br></p>