Sql – LINQ to SQL delete producing “Specified cast is not valid” error

I've got a painfully simple table that is giving me a "Specified cast is not valid" error when I try to delete one or more rows. The table has two columns, an "id" as the primary key (INT), and a "name" (VARCHAR(20)), which maps to a String in the LINQ to SQL dbml file. Both of these statements produce the error:

dc.DeleteOnSubmit(dc.MyTables.Where(Function(x) x.id = 1).SingleOrDefault)
dc.DeleteAllOnSubmit(dc.MyTables)

I iterated through "MyTable" just to make sure there was no weird data, and there are only two rows:

• id = 1, name = "first"
• id = 2, name = "second"

The exception is happening on SubmitChanges. Here is the stack trace:

[InvalidCastException: Specified cast is not valid.]
   System.Data.Linq.SingleKeyManager`2.TryCreateKeyFromValues(Object[] values, V& v) +59
   System.Data.Linq.IdentityCache`2.Find(Object[] keyValues) +28
   System.Data.Linq.StandardIdentityManager.Find(MetaType type, Object[] keyValues) +23
   System.Data.Linq.CommonDataServices.GetCachedObject(MetaType type, Object[] keyValues) +48
   System.Data.Linq.ChangeProcessor.GetOtherItem(MetaAssociation assoc, Object instance) +142
   System.Data.Linq.ChangeProcessor.BuildEdgeMaps() +233
   System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode) +59
   System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode) +331
   System.Data.Linq.DataContext.SubmitChanges() +19
   InpatientCensus.MaintenanceController.DeleteSoleCommunity(Int32 id) in C:\Documents and Settings\gregf\My Documents\Projects\InpatientCensus\InpatientCensus\Controllers\MaintenanceController.vb:14
   lambda_method(ExecutionScope , ControllerBase , Object[] ) +128
   System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +17
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +178
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +24
   System.Web.Mvc.<>c__DisplayClassa.<InvokeActionMethodWithFilters>b__7() +52
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +254
   System.Web.Mvc.<>c__DisplayClassc.<InvokeActionMethodWithFilters>b__9() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +192
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +399
   System.Web.Mvc.Controller.ExecuteCore() +126
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +27
   System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +7
   System.Web.Mvc.MvcHandler.ProcessRequest(HttpContextBase httpContext) +151
   System.Web.Mvc.MvcHandler.ProcessRequest(HttpContext httpContext) +57
   System.Web.Mvc.MvcHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext httpContext) +7
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

Removing the association added to the DBML file allows rows to be deleted. Why would the association be causing the error? The associated columns are both VARCHAR(20) in the database and resolve to Strings in the DBML file.

What could possibly be causing a casting error?