Computed Field In Typed Dataset

I have a list box that is bound to an object data source. I added a column,
say ColumnC, to my typed data set (using the designer) and for the expression
property I entered

ColumnA + ': ' + ColumnB

When I preview the data in the designer I get exactly what I want.

Now I go over to the object data source on my web page, refresh it and
refresh the schema. I can now see ColumnC as an option for the list box's
data text field. However, when I run the page nothing shows up in the list
box. However, when I view the page source, I see that the data value field
did render.

Next I put in a value (instead of the default DBNull) for the DefaultValue
property. After going through the whole exercise of refreshing and such
again, I run the page and the default value I put in is returned in the list
box.

Does anyone know what I missed? It must be something stupid.

Thanks ...