Newbie - stuck - please Help

I have probably a very basic error, but I don't know how to get this to work.

I am trying to put a combo box on a form which includes values from a data base - firstName+' '+lastName, sorted by the lastName. I added the field to the dataDesigner and clicked on the smart tag. Went into query builder and when I execute the query, the correct fields are showing, but when I click OK to go back to the form, I'm getting an error that I don't understand. The error says: "The schema returned by the new query differs from the base query."

I thought that was the whole point in going into the query builder. This error won't let me do anything. Did I forget to do something somewhere else? Please help..:blush: