When invoke the expression, i become an InvalidOperationException

variable ‘___zip_0’ of type ‘System.Int32’ referenced from scope ”, but it is not defined

public class AddressByZipSpecification : Specification<Address>
{
    public int _zip { get; set; }
    public AddressByZipSpecification(int zip)
    {
        _zip = zip;
    }
    public override Expression<Func<Address, bool>> ToExpression()
    {
        return address => address.Zip == _zip;
    }
}



var zipSpec = new AddressByZipSpecification(12);
var address = await dbContext.Address.Where(a =>  zipSpec.ToExpression().Invoke(a)).ToListAsync();

I am at a loss, what happend in the expression tree