diff --git a/Libplanet.Tests/Net/SwarmTest.cs b/Libplanet.Tests/Net/SwarmTest.cs
index 3c1a346c72d..56a2604847d 100644
--- a/Libplanet.Tests/Net/SwarmTest.cs
+++ b/Libplanet.Tests/Net/SwarmTest.cs
@@ -274,51 +274,6 @@ public async Task CanExchangePeer()
             }
         }
 
-        [Fact(Timeout = Timeout)]
-        public async Task WorksAsCollection()
-        {
-            Swarm a = _swarms[0];
-            Swarm b = _swarms[1];
-            Swarm c = _swarms[2];
-
-            // Obtaining swarm's endpoint...
-            await Task.WhenAll(
-                StartAsync(a, _blockchains[0]),
-                StartAsync(b, _blockchains[1]),
-                StartAsync(c, _blockchains[2]));
-
-            Assert.Empty(a);
-            Assert.Empty(b);
-            Assert.Empty(c);
-
-            a.Add(b.AsPeer);
-            a.Add(c.AsPeer);
-            Assert.Contains(b.AsPeer, a);
-            Assert.Contains(c.AsPeer, a);
-
-            Peer[] peers = null;
-            Assert.Throws<ArgumentNullException>(() =>
-            {
-                a.CopyTo(peers, 0);
-            });
-
-            peers = new Peer[3];
-            Assert.Throws<ArgumentOutOfRangeException>(() =>
-            {
-                a.CopyTo(peers, -1);
-            });
-            Assert.Throws<ArgumentException>(() =>
-            {
-                a.CopyTo(peers, 2);
-            });
-
-            a.CopyTo(peers, 1);
-
-            Assert.Equal(
-                new HashSet<Peer> { null, b.AsPeer, c.AsPeer },
-                peers.ToHashSet());
-        }
-
         [Fact(Timeout = Timeout)]
         public async Task DetectAppProtocolVersion()
         {
@@ -353,8 +308,8 @@ public async Task DetectAppProtocolVersion()
 
                 foreach (var peer in peers)
                 {
-                    a.Add(peer);
-                    b.Add(peer);
+                    await a.AddPeersAsync(new[] { peer });
+                    await b.AddPeersAsync(new[] { peer });
                 }
 
                 Assert.Equal(new[] { c.AsPeer }, a.ToArray());
@@ -396,7 +351,7 @@ void GameHandler(object sender, DifferentProtocolVersionEventArgs e)
                 await StartAsync(a, chain);
                 await StartAsync(b, chain);
 
-                a.Add(b.AsPeer);
+                await a.AddPeersAsync(new[] { b.AsPeer });
 
                 Assert.True(isCalled);
             }
@@ -771,7 +726,7 @@ public async Task InitialBlockDownload()
             try
             {
                 await StartAsync(minerSwarm, minerChain);
-                receiverSwarm.Add(minerSwarm.AsPeer);
+                await receiverSwarm.AddPeersAsync(new[] { minerSwarm.AsPeer });
 
                 await StartAsync(receiverSwarm, receiverChain);
 
@@ -813,7 +768,7 @@ public async Task Preload()
             try
             {
                 await StartAsync(minerSwarm, minerChain);
-                receiverSwarm.Add(minerSwarm.AsPeer);
+                await receiverSwarm.AddPeersAsync(new[] { minerSwarm.AsPeer });
 
                 await receiverSwarm.PreloadAsync(receiverChain, progress);
 
diff --git a/Libplanet/Net/Swarm.cs b/Libplanet/Net/Swarm.cs
index f33079891ca..3bb554480c6 100644
--- a/Libplanet/Net/Swarm.cs
+++ b/Libplanet/Net/Swarm.cs
@@ -252,7 +252,7 @@ public async Task<ISet<Peer>> AddPeersAsync(
             DateTimeOffset? timestamp = null,
             CancellationToken cancellationToken = default(CancellationToken))
         {
-            if (timestamp == null)
+            if (timestamp is null)
             {
                 timestamp = DateTimeOffset.UtcNow;
             }
@@ -315,6 +315,10 @@ public async Task<ISet<Peer>> AddPeersAsync(
                         );
                     }
                 }
+                else
+                {
+                    _peers[peer] = timestamp.Value;
+                }
             }
 
             return addedPeers;