diff --git a/CHANGES.md b/CHANGES.md
index e83fd2af23b..efe7530e3be 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -121,6 +121,8 @@ To be released.
     constructor.  [[#1012]]
  -  Added `cacheSize` optional parameter to `BlockSet<T>()` constructor.
     [[#1013]]
+ -  Removed `Address(SerializationInfo, StreamingContext)` constructor.
+    [[#1022]]
  -  Removed constructors from `InvalidMessageException` class.  [[#1021]]
 
 ### Backward-incompatible network protocol changes
@@ -366,6 +368,7 @@ To be released.
 [#1012]: https://github.com/planetarium/libplanet/pull/1012
 [#1013]: https://github.com/planetarium/libplanet/pull/1013
 [#1021]: https://github.com/planetarium/libplanet/pull/1021
+[#1022]: https://github.com/planetarium/libplanet/pull/1022
 [sleep mode]: https://en.wikipedia.org/wiki/Sleep_mode
 
 
diff --git a/Libplanet/Address.cs b/Libplanet/Address.cs
index 6aac754e519..694e50b321b 100644
--- a/Libplanet/Address.cs
+++ b/Libplanet/Address.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics.Contracts;
@@ -93,13 +94,6 @@ public Address(byte[] address)
         {
         }
 
-        public Address(
-            SerializationInfo info,
-            StreamingContext context)
-            : this(info.GetValue<byte[]>("address"))
-        {
-        }
-
         /// <summary>
         /// Derives the corresponding <see cref="Address"/> from a <see
         /// cref="PublicKey"/>.
@@ -138,6 +132,14 @@ public Address(string hex)
         {
         }
 
+        private Address(
+            SerializationInfo info,
+            StreamingContext context)
+            : this(info?.GetValue<byte[]>("address") ??
+                throw new SerializationException("Missing the address field."))
+        {
+        }
+
         /// <summary>
         /// An immutable array of 20 <see cref="byte"/>s that represent this
         /// <see cref="Address"/>.
@@ -223,9 +225,7 @@ public override string ToString()
         }
 
         /// <inheritdoc />
-        public void GetObjectData(
-            SerializationInfo info,
-            StreamingContext context)
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
         {
             info.AddValue("address", ToByteArray());
         }
@@ -246,7 +246,7 @@ int IComparable<Address>.CompareTo(Address other)
             return 0;
         }
 
-        int IComparable.CompareTo(object obj)
+        int IComparable.CompareTo(object? obj)
         {
             if (obj is Address other)
             {
diff --git a/Libplanet/AddressExtensions.cs b/Libplanet/AddressExtensions.cs
index c6c73785eaa..1e4acccdbdb 100644
--- a/Libplanet/AddressExtensions.cs
+++ b/Libplanet/AddressExtensions.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using Libplanet.Crypto;
 
 namespace Libplanet
diff --git a/Libplanet/ByteArrayExtensions.cs b/Libplanet/ByteArrayExtensions.cs
index 7e414996d4e..67431264731 100644
--- a/Libplanet/ByteArrayExtensions.cs
+++ b/Libplanet/ByteArrayExtensions.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Diagnostics.Contracts;
 
@@ -22,7 +23,7 @@ public static class ByteArrayExtensions
         /// <paramref name="bytes"/> or <paramref name="prefix"/> is null.
         /// </exception>
         [Pure]
-        public static bool StartsWith(this byte[] bytes, byte[] prefix)
+        public static bool StartsWith(this byte[]? bytes, byte[]? prefix)
         {
             if (bytes is null)
             {
diff --git a/Libplanet/ByteUtil.cs b/Libplanet/ByteUtil.cs
index 26eb115c42a..7a1b1730e30 100644
--- a/Libplanet/ByteUtil.cs
+++ b/Libplanet/ByteUtil.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics.Contracts;
@@ -74,7 +75,6 @@ public static string Hex(byte[] bytes)
             }
 
             string s = BitConverter.ToString(bytes);
-
             return s.Replace("-", string.Empty).ToLower(CultureInfo.InvariantCulture);
         }
 
diff --git a/Libplanet/FixedSizedQueue.cs b/Libplanet/FixedSizedQueue.cs
index 30f6baf6ea0..ff2943f4ae5 100644
--- a/Libplanet/FixedSizedQueue.cs
+++ b/Libplanet/FixedSizedQueue.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using System.Collections.Concurrent;
 
 namespace Libplanet
diff --git a/Libplanet/HashDigest.cs b/Libplanet/HashDigest.cs
index e6ff2aeb180..82b75c9604f 100644
--- a/Libplanet/HashDigest.cs
+++ b/Libplanet/HashDigest.cs
@@ -1,8 +1,10 @@
+#nullable enable
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics.Contracts;
 using System.Linq;
 using System.Numerics;
+using System.Reflection;
 using System.Security.Cryptography;
 
 namespace Libplanet
@@ -35,8 +37,9 @@ namespace Libplanet
 
         static HashDigest()
         {
-            var thunk = (T)typeof(T).GetMethod("Create", new Type[0]).Invoke(
-                null, new object[0]);
+            MethodInfo? method = typeof(T).GetMethod("Create", new Type[0]);
+            T thunk = method?.Invoke(null, new object[0]) as T
+                ?? throw new InvalidCastException($"Failed to instantiate {typeof(T).FullName}.");
             Size = thunk.HashSize / 8;
 
             _defaultByteArray = new byte[Size];
@@ -206,7 +209,7 @@ public override string ToString()
         }
 
         [Pure]
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             return obj is IEquatable<HashDigest<T>> other
                 ? other.Equals(this)
diff --git a/Libplanet/Hashcash.cs b/Libplanet/Hashcash.cs
index e4ec9a8725d..3e164f59323 100644
--- a/Libplanet/Hashcash.cs
+++ b/Libplanet/Hashcash.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Security.Cryptography;
 using System.Threading;
diff --git a/Libplanet/Nonce.cs b/Libplanet/Nonce.cs
index d3d89eb838a..f4f72802770 100644
--- a/Libplanet/Nonce.cs
+++ b/Libplanet/Nonce.cs
@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Immutable;
 using System.Diagnostics.Contracts;