Csharp MongoDB实体框架核心：表达必须可写

我正在将实体框架核心与MongoDB一起使用.然而，我在try 使用FirstOrDefault时遇到了一个问题.

我有一个基本类，SignInCredentials:

public class SignInCredentials(string username, string? password)
{
    public string Username { get; set; } = username;
    public string? Password { get; protected init; } = password;
}

我将其扩展为Account:

public class Account : SignInCredentials
{
    // [Key] sets the primary key
    [Key]
    public ObjectId Id { get; private init; }

    private readonly byte[] salt;

    public Account(string username, string password) : base(username, null)
    {
        Id = new ObjectId();
        salt = GenerateSalt();
        Password = Hash(password, salt);
    }

    // More code not displayed
}

我存储一个DbSet<Account>，配置为: modelBuilder.Entity<Account>().ToCollection("accounts");

然而，当我try :accounts.FirstOrDefault(a => a.Username == creds.Username);(其中creds是SignInCredentials)时，我得到以下错误:

System.ArgumentException: Expression must be writeable (Parameter 'left')
         at System.Linq.Expressions.Expression.RequiresCanWrite(Expression expression, String paramName)
         at System.Linq.Expressions.Expression.Assign(Expression left, Expression right)
         at System.Linq.Expressions.Expression.MakeBinary(ExpressionType binaryType, Expression left, Expression right, Boolean liftToNull, MethodInfo method, LambdaExpression conversion)
         at System.Linq.Expressions.BinaryExpression.Update(Expression left, LambdaExpression conversion, Expression right)
         at System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression node)
         at MongoDB.EntityFrameworkCore.Query.Visitors.ProjectionBindingRemovingExpressionVisitor.VisitBinary(BinaryExpression binaryExpression)

如何修复此错误？