Minor edits

src/Discord.Net/API/Responses.cs

@@ -102,7 +102,7 @@ namespace Discord.API
 			[JsonProperty("url")]
 			public string Url;
 			[JsonProperty("updated-at")]
-			public DateTime UpdatedAt;
+			public DateTime? UpdatedAt;
 		}
 
 		public sealed class MaintenanceData

src/Discord.Net/Models/Member.cs

@@ -39,7 +39,7 @@ namespace Discord
 		/// <summary> Returns the time this user last sent/edited a message, started typing or sent voice data in this server. </summary>
 		public DateTime? LastActivityAt { get; private set; }
 		/// <summary> Returns the time this user was last seen online in this server. </summary>
-		public DateTime? LastOnlineAt => Status != UserStatus.Offline ? DateTime.UtcNow : _lastOnline;
+		public DateTime LastOnlineAt => Status != UserStatus.Offline ? DateTime.UtcNow : _lastOnline;
 		private DateTime _lastOnline;
 
 		public string UserId { get; }