How can this Linq2Sql create an enum in the select clause?

public IQueryable<Models.Achievement> GetAchievements()
{
    return from a in _sqlDatabase.Achievements
        select new Models.Achievement
            {
                // Note: ToEnum is an extension method that converts an int -> the enum.
                AchievementType = a.AchievementTypeId.ToEnum<Models.AchievementType>(),
                DateTimeCreated = a.DateTimeCreated,
                UserId = a.UserId
            };
}

System.NotSupportedException: Method 'Models.AchievementType
    ToEnum[AchievementType](int)' has no supported translation to SQL.

select new {..., AchievementType = (AchievementType) foo.Bar, ...}

AchievementType achievementType{
    get
      {
         return this.AchievementTypeId.ToEnum<Models.AchievementType>();
      }
      set
      {
        this.AchievementTypeId = (int)value;
      }
    }