It is not possible to tell from your screenshot what the item is, but it is possibly a content control that is showing its prompt text, i.e. no value has been selected. You can check for this by modifying your code:
Code:
If Not comboBox Is Nothing Then
If comboBox.Type = wdContentControlComboBox Then
If comboBox.ShowingPlaceholderText Then
'no value has been selected
Else: wert = comboBox.Range.Text
End If