unable to create a constant value of type

In the first, my project is OK.

A few days ago, I add some entity to database (use Entity Framework 6.0, Code First), everystep is OK.

And then, it occurred this problem.

The Example Code is:

class A
{
    public int Id{get;set;}
}

class B
{
    public int Id{get;set;}
    public int Aid{get;set;}
    public virtual A A{get;set;}
}

//下面的Linq就报告错误：

Bs.Where(x=>x.A==a).FirstOrDefault();