Entity Framework is throwing an error "Unable to evaluate the expression. Operation not supported. Unknown error: 0x80070057."

I am using Entity Framework to get db data. I wrote an action to return table data in JSON format as below:

public JsonResult GetEmployeesData()
{
     using (TrainingDBEntities db = new TrainingDBEntities())
     {
           return new JsonResult { Data = db.Employees, JsonRequestBehavior = JsonRequestBehavior.AllowGet };
     }
}

It's not throwing any exception. But a console error is occurring with error code: 500. When I debugged, it showed an error

The function evaluation requires all threads to run

When I try to reload, a new error occurs:

Unable to evaluate the expression. Operation not supported. Unknown error: 0x80070057

I don't have any idea what's wrong with this code?

The solution is :

public JsonResult GetEmployeesData()
{
     using (TrainingDBEntities db = new TrainingDBEntities())
     {
           var emps = db.Employees.ToList();
           return new JsonResult { Data = emps, JsonRequestBehavior = JsonRequestBehavior.AllowGet };
     }
}

I think accessing db inside the JsonResult{} is causing the issue.