How can I use SumAsync to calculate sum of a customized value object in Asp.Net core

Question

I have a Model with a property of a value object type as following:

public class Course : AggregateRoot, ISpModel
{
    ...
    public UnsignedNumber MaximumCapacity { get; private set; }
    ...
}

with UnsignedNumber being a value object containing a short value:

public class UnsignedNumber : BaseValueObject<UnsignedNumber>
{
    public short Value { get; }
    ...
}

What I need to do is to sum all the MaximumCapacities of courses which correspond with certain conditions, but when I try to add a SumAsync(x => x.MaximumCapacity) at the end of the query, I get a syntax error the syntax error and when I try to do the same with it's value, I get a linq error in runtime.

"The LINQ expression '(int)(EntityShaperExpression: \r\n EntityType: Course\r\n ValueBufferExpression: \r\n (ProjectionBindingExpression: Outer)\r\n IsNullable: False\r\n).MaximumCapacity.Value' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information."

Edit: Here's the Linq Expression that encounters the problem:

var query = _dbContext.Courses.AsQueryable();
query = query.Include(x => BunchOfIncludes(x));
var res = await query.Where(x => BunchOfClauses(x)).SumAsync(x => x.MaximumCapacity.Value);

Edit2: the classes mentioned above:

public abstract class AggregateRoot : Entity
{
    private readonly List<IDomainEvent> _events;
    protected AggregateRoot() => _events = new List<IDomainEvent>();
    public AggregateRoot(IEnumerable<IDomainEvent> events)
    {
        if (events == null) return;
        foreach (var @event in events)
            ((dynamic)this).On((dynamic)@event);
    }
    protected void AddEvent(IDomainEvent @event) => _events.Add(@event);
    public IEnumerable<IDomainEvent> GetEvents() => _events.AsEnumerable();
    public void ClearEvents() => _events.Clear();
}

public interface ISpModel
{
}

public abstract class BaseValueObject<TValueObject> : IEquatable<TValueObject>
        where TValueObject : BaseValueObject<TValueObject>
{
...
    public static bool operator ==(BaseValueObject<TValueObject> right, BaseValueObject<TValueObject> left)
    {
        if (right is null && left is null)
            return true;
        if (right is null || left is null)
            return false;
        return right.Equals(left);
    }
...
}