Issue in LINQ to Sharepoint 2010

You see this error, when you try to retrieve an item or many item in LINQ ( context.List.ScopeToFolder("",true).FirstOrDefault() or context.List.ScopeToFolder("",true).ToList() ... ):

Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack

please turn on BrowseDirectories role

public static void UpdateReadPermissionLevel(SPWeb web)

{

SPRoleDefinition role = web.RoleDefinitions["Read"];

role.BasePermissions = role.BasePermissions | SPBasePermissions.BrowseDirectories;

role.Update();

}