I have seen this a few times - and the only way to fix it seems to be to remove all cookies for the site.

It seems to happen when the site has been left midway through the shopping basket phase. The full error message is:

Server Error in '/' Application.

Nullable object must have a value.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidOperationException: Nullable object must have a value.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[InvalidOperationException: Nullable object must have a value.]
   System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) +52
   TeaCommerce.SessionController.get_CurrentOrder() +251
   TeaCommerce.Razor.TeaCommerce.GetOrder() +4
   Printed4uCMS.Masterpages.Site.SetupPage() +439
   Printed4uCMS.Masterpages.Site.Page_Load(Object sender, EventArgs e) +77
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +91
   System.Web.UI.Control.LoadRecursive() +74
   System.Web.UI.Control.LoadRecursive() +146
   System.Web.UI.Control.LoadRecursive() +146
   System.Web.UI.Control.LoadRecursive() +146
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2207

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.272

Any ideas what this is?