@JMerk, if I had to guess:
1. You have customised a DataCard and inserted in a Combo Box
2. The field to be saved with the combo box selection is a Text data type in your source data
3. The Default property of the DataCard (not the DataCardValue) has not been changed. So the expression should be something like:
This.'Your Text Field'
4. The Update property of the DataCard is set to your ComboBox selection. E.g.
ComboBox1.Selected.Value​
5. The Combo Box has been configured to be single select
Assuming all the above because of the lack of detail you have provided, try:
{Value: Parent.Default}