Noob DetailView/FilterExpression Question (C#, Visual Studio 2010)

Question

I have a DetailView (RunnerDetails) bound to RunnerDataSource, and I set the FilterExpression of that DataSource from user input:

        String WhereClause = FindBox.Text;
        RunnerDataSource.FilterExpression = WhereClause;
        RunnerDetails.DataBind();

The DetailView then immediately re-displays with the correct info, but when I use the "Edit" method the edited record is the first in the dataset. I've tried DetailView.DataBind and everything else I can think of and always get the same behavior, so I think I'm missing something obvious.