Passig DistinctBy to Generic Repository as a parameter

Question

I'm using the Generic Repository pattern on my project and I have the following method that retrieves data according to the parameters provided :

public virtual async Task<IReadOnlyCollection<TEntity>> GetAsync(
   Expression<Func<TEntity, bool>> filter = null, Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null)
{
    IQueryable<TEntity> query = dbSet;

    if (filter != null)
    {
        query = query.Where(filter);
    }

    if (orderBy != null)
    {
        query = orderBy(query);
    }

    return await query.ToListAsync();
    

And I'm trying to add another paramter to apply a DistinctBy filter and this's what I did :

public virtual async Task<IReadOnlyCollection<TEntity>> GetAsync(
   Expression<Func<TEntity, bool>> filter = null, Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null,
   Expression<Func<TEntity, object>> distinctBy = null)
{
    IQueryable<TEntity> query = dbSet;

    if (filter != null)
    {
        query = query.Where(filter);
    }

    if (orderBy != null)
    {
        query = orderBy(query);
    }

    if (distinctBy != null)
    {
        query = query.DistinctBy(distinctBy);
    }

    return await query.ToListAsync();
}

But it's not working and I dont understand what I'm doing wrong. Thsi gives me the following exception :

The LINQ expression 'DbSet...' 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 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'

Thank you.

Edit :

I created a new class in which i put the DistinctBy extension method I'm going to use :

public static class EntityFrameWorkExtensions
{
    public static IQueryable<TSource> DistinctByPlus<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector)
    {
        return source.GroupBy(keySelector)?.Select(x => x.FirstOrDefault());
    }
}

And on my GenericRepository class I edited the signature of my method and I used this new extension method like this :

 public virtual async Task<IReadOnlyCollection<TEntity>> GetAsync(
   Expression<Func<TEntity, bool>> filter = null, Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null,
   Expression<Func<TEntity, object>> distinctBy = null)
{
    IQueryable<TEntity> query = dbSet;

    if (filter != null)
    {
        query = query.Where(filter);
    }

    if (orderBy != null)
    {
        query = orderBy(query);
    }

    if (distinctBy != null)
    {
        query = query.DistinctByPlus(distinctBy);
    }
    
    int count = query.Count();
    return await query.ToListAsync();
}

Here's my call to this method :

var data = await _unitOfWork.UserRepository.GetAsync(x => x.userId == id, distinctBy: o => o.SupervisorId);

But now this's giving me another error :

Nullable object must have a value.

This exception is thrown exactly after int count = query.Count();. I know this line looks useless but I will need it later.

Even though none of my database table records have SupervisorId null and when I remove the distinctBy the call works perfectly.