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