Cannot insert the value NULL into column '', table 'C:\USERS\'; column does not allow nulls. UPDATE fails.

I'm a career-long ASP.NET fan, but I'm getting close to giving up on it. The above error has plagued me in one way or another for about a month now.  Basically, what I'm trying to do is use SQLDatasources to interact with my database via a web-based front end, and it just shouldn't be this hard.  I create a datasource like this:
<asp:SqlDataSource ID="sqlDS_DeliveryOptions" runat="server" ConnectionString="<%$ ConnectionStrings:prospectsConnectionString %>" SelectCommand="SELECT latencyRanges.latencyRequirement, latencyRanges.ID AS latID, bandwidthRanges.bandwidthRequirement, bandwidthRanges.ID AS bwID, commProviders.communicationsProvider, commProviders.ID AS commProID, dataSourceMethods.dataSourceMethod, dataSourceMethods.ID AS dsmID, APIs.API, APIs.ID AS apiID, deliveryOptions.interestedInDeployedServices, deliveryOptions.ProjectID FROM bandwidthRanges INNER JOIN deliveryOptions ON bandwidthRanges.ID = delivery ...