CompiledQuery throws an ArgumentException

I am trying to use a very simple precompiled query in an extension method to retrieve data using LINQ to SQL. SytelineRepository is my custom DataContext that uses external mapping files, and I'm querying the JobOrders table. For a long time, I used a custom DataContext without problems and executed the same request without problems; I'm just trying to improve performance. However, when I try to use CompiledQuery.Compile, I get an ArgumentException below.

Here is the code:

public static class Queries
{
    public static readonly Func<SytelineRepository, String, IQueryable<JobOrder>> GetOpenJobOrdersForItemQuery =
        CompiledQuery.Compile(
        (SytelineRepository r, String itemNumber) => 
            from job in r.JobOrders
            where job.ItemNumber == itemNumber
            select job
        );

    public static IQueryable<JobOrder> GetOpenJobOrdersForItem(this SytelineRepository r, System.String itemNumber)
    {
        return GetOpenJobOrdersForItemQuery(r, itemNumber);
    }

}

In contrast, this works:

    public static IEnumerable<JobOrder> GetOpenJobOrdersForItem(this SytelineRepository r, System.String itemNumber)
    {
        return GetOpenJobOrdersForItemUncompiledQuery(r, itemNumber);
    }

    public static IQueryable<JobOrder> GetOpenJobOrdersForItemUncompiledQuery(SytelineRepository r, String itemNumber)
    {
        return
            from job in r.JobOrders
            where job.ItemNumber == itemNumber
            select job;
    }

Here's the complete error:

System.ArgumentException fix was unhandled

= 'System.String ItemNumber' 'System.Linq.IQueryable`1 [Mpicorp.SytelineDataModel.JobOrder]'

   at System.Linq.Expressions.Expression.Property(Expression expression, PropertyInfo property)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.AccessMember(SqlMember m, SqlExpression expo)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitBinaryOperator(SqlBinary bo)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitExpression(SqlExpression expr)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitSelect(SqlSelect select)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Visitor.VisitIncludeScope(SqlIncludeScope scope)
   at System.Data.Linq.SqlClient.SqlVisitor.Visit(SqlNode node)
   at System.Data.Linq.SqlClient.SqlBinder.Bind(SqlNode node)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(ResultShape resultShape, Type resultType, SqlNode node, ReadOnlyCollection`1 parentParameters, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.BuildQuery(Expression query, SqlNodeAnnotations annotations)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Compile(Expression query)
   at System.Data.Linq.CompiledQuery.ExecuteQuery(DataContext context, Object[] args)
   at System.Data.Linq.CompiledQuery.Invoke[TArg0,TArg1,TResult](TArg0 arg0, TArg1 arg1)
   at Mpicorp.SytelineDataModel.Queries.GetOpenJobOrdersForItem(SytelineRepository r, String itemNumber) in C:\SVN\Mpicorp.SytelineDataModel\trunk\SytelineDataModel\Queries.cs:line 21
   at Mpicorp.SytelineDataModel.SytelineRepository.GetTimePhasedInventory(String itemNumber, Boolean includeForecast) in C:\SVN\Mpicorp.SytelineDataModel\trunk\SytelineDataModel\SytelineRepository.cs:line 242
   at RepriceWorkbench.TimePhasedInventoryForm..ctor(SytelineRepository repository, String itemNumber) in C:\SVN\spirepriceutility\trunk\src\POWorkbench\TimePhasedInventoryForm.cs:line 32
   at RepriceWorkbench.TimePhasedMenuForm.goBtn_Click(Object sender, EventArgs e) in C:\SVN\spirepriceutility\trunk\src\POWorkbench\TimePhasedMenuForm.cs:line 25
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at RepriceWorkbench.Program.Main() in C:\SVN\spirepriceutility\trunk\src\POWorkbench\Program.cs:line 66
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()