diff --git a/Libplanet.Explorer/Controllers/ExplorerController.cs b/Libplanet.Explorer/Controllers/ExplorerController.cs
index e330fc7d0e4..bc340df3e72 100644
--- a/Libplanet.Explorer/Controllers/ExplorerController.cs
+++ b/Libplanet.Explorer/Controllers/ExplorerController.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using GraphQL.SystemTextJson;
diff --git a/Libplanet.Explorer/Controllers/GraphQLBody.cs b/Libplanet.Explorer/Controllers/GraphQLBody.cs
index f5d6de3a5f5..e61baa96511 100644
--- a/Libplanet.Explorer/Controllers/GraphQLBody.cs
+++ b/Libplanet.Explorer/Controllers/GraphQLBody.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using Newtonsoft.Json.Linq;
 
 namespace Libplanet.Explorer.Controllers
diff --git a/Libplanet.Explorer/ExplorerStartup.cs b/Libplanet.Explorer/ExplorerStartup.cs
index 129ceed6b93..45f4ae2416f 100644
--- a/Libplanet.Explorer/ExplorerStartup.cs
+++ b/Libplanet.Explorer/ExplorerStartup.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using Libplanet.Action;
 using Libplanet.Explorer.Controllers;
 using Libplanet.Explorer.Interfaces;
diff --git a/Libplanet.Explorer/GraphTypes/ActionType.cs b/Libplanet.Explorer/GraphTypes/ActionType.cs
index 11c396cfb1d..5c81d60bee5 100644
--- a/Libplanet.Explorer/GraphTypes/ActionType.cs
+++ b/Libplanet.Explorer/GraphTypes/ActionType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using System.Collections.Generic;
 using System.Linq;
diff --git a/Libplanet.Explorer/GraphTypes/AddressType.cs b/Libplanet.Explorer/GraphTypes/AddressType.cs
index 2c6a5b032bc..a384cd75cf4 100644
--- a/Libplanet.Explorer/GraphTypes/AddressType.cs
+++ b/Libplanet.Explorer/GraphTypes/AddressType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using GraphQL.Language.AST;
 using GraphQL.Types;
diff --git a/Libplanet.Explorer/GraphTypes/BlockType.cs b/Libplanet.Explorer/GraphTypes/BlockType.cs
index 56958569d4c..ede66eeb65d 100644
--- a/Libplanet.Explorer/GraphTypes/BlockType.cs
+++ b/Libplanet.Explorer/GraphTypes/BlockType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System.Security.Cryptography;
 using GraphQL.Types;
 using Libplanet.Action;
diff --git a/Libplanet.Explorer/GraphTypes/ByteStringType.cs b/Libplanet.Explorer/GraphTypes/ByteStringType.cs
index 7b79ea791b1..4d12970a4d8 100644
--- a/Libplanet.Explorer/GraphTypes/ByteStringType.cs
+++ b/Libplanet.Explorer/GraphTypes/ByteStringType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using GraphQL.Language.AST;
 using GraphQL.Types;
diff --git a/Libplanet.Explorer/GraphTypes/NodeStateType.cs b/Libplanet.Explorer/GraphTypes/NodeStateType.cs
index 3f13d2382f7..80c92c5fc70 100644
--- a/Libplanet.Explorer/GraphTypes/NodeStateType.cs
+++ b/Libplanet.Explorer/GraphTypes/NodeStateType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using GraphQL.Types;
 using Libplanet.Action;
 using Libplanet.Explorer.Interfaces;
diff --git a/Libplanet.Explorer/GraphTypes/PublicKeyType.cs b/Libplanet.Explorer/GraphTypes/PublicKeyType.cs
index 0591372a2ab..32b52dd9fc3 100644
--- a/Libplanet.Explorer/GraphTypes/PublicKeyType.cs
+++ b/Libplanet.Explorer/GraphTypes/PublicKeyType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using GraphQL.Language.AST;
 using GraphQL.Types;
diff --git a/Libplanet.Explorer/GraphTypes/TransactionType.cs b/Libplanet.Explorer/GraphTypes/TransactionType.cs
index da6b54ae9f0..fe81fecd2ab 100644
--- a/Libplanet.Explorer/GraphTypes/TransactionType.cs
+++ b/Libplanet.Explorer/GraphTypes/TransactionType.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System.Linq;
 using GraphQL;
 using GraphQL.Types;
diff --git a/Libplanet.Explorer/Interfaces/IBlockChainContext.cs b/Libplanet.Explorer/Interfaces/IBlockChainContext.cs
index 3d0a5a9f711..b65c0098a9d 100644
--- a/Libplanet.Explorer/Interfaces/IBlockChainContext.cs
+++ b/Libplanet.Explorer/Interfaces/IBlockChainContext.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System.Runtime.CompilerServices;
 using GraphQL.Types;
 using Libplanet.Action;
diff --git a/Libplanet.Explorer/Libplanet.Explorer.csproj b/Libplanet.Explorer/Libplanet.Explorer.csproj
index e32bd16c851..a82a63be74c 100644
--- a/Libplanet.Explorer/Libplanet.Explorer.csproj
+++ b/Libplanet.Explorer/Libplanet.Explorer.csproj
@@ -20,6 +20,7 @@
     <RootNamespace>Libplanet.Explorer</RootNamespace>
     <AssemblyName>Libplanet.Explorer</AssemblyName>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+    <Nullable>enable</Nullable>
     <CheckForOverflowUnderflow>true</CheckForOverflowUnderflow>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <IsTestProject>false</IsTestProject>
diff --git a/Libplanet.Explorer/Queries/BlockQuery.cs b/Libplanet.Explorer/Queries/BlockQuery.cs
index cda18cb8b0e..6da15bf5059 100644
--- a/Libplanet.Explorer/Queries/BlockQuery.cs
+++ b/Libplanet.Explorer/Queries/BlockQuery.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using GraphQL;
 using GraphQL.Types;
 using Libplanet.Action;
diff --git a/Libplanet.Explorer/Queries/ExplorerQuery.cs b/Libplanet.Explorer/Queries/ExplorerQuery.cs
index c257bfe554a..96ebaf18982 100644
--- a/Libplanet.Explorer/Queries/ExplorerQuery.cs
+++ b/Libplanet.Explorer/Queries/ExplorerQuery.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using System.Collections.Generic;
 using System.Linq;
diff --git a/Libplanet.Explorer/Queries/TransactionQuery.cs b/Libplanet.Explorer/Queries/TransactionQuery.cs
index f162f6cf133..b113ba8df6d 100644
--- a/Libplanet.Explorer/Queries/TransactionQuery.cs
+++ b/Libplanet.Explorer/Queries/TransactionQuery.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using GraphQL;
 using GraphQL.Types;
diff --git a/Libplanet.Explorer/Store/AddressRefDoc.cs b/Libplanet.Explorer/Store/AddressRefDoc.cs
index 445caa1e499..46564c76457 100644
--- a/Libplanet.Explorer/Store/AddressRefDoc.cs
+++ b/Libplanet.Explorer/Store/AddressRefDoc.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using Libplanet.Tx;
 using LiteDB;
diff --git a/Libplanet.Explorer/Store/IRichStore.cs b/Libplanet.Explorer/Store/IRichStore.cs
index 5b0f5a4d198..f3533358616 100644
--- a/Libplanet.Explorer/Store/IRichStore.cs
+++ b/Libplanet.Explorer/Store/IRichStore.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using System.Collections.Generic;
 using Libplanet.Blocks;
diff --git a/Libplanet.Explorer/Store/LiteDBRichStore.cs b/Libplanet.Explorer/Store/LiteDBRichStore.cs
index 331320ff098..4dbabbd8d48 100644
--- a/Libplanet.Explorer/Store/LiteDBRichStore.cs
+++ b/Libplanet.Explorer/Store/LiteDBRichStore.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
diff --git a/Libplanet.Explorer/Store/MySQLRichStore.cs b/Libplanet.Explorer/Store/MySQLRichStore.cs
index 227e86609d1..7f304228f32 100644
--- a/Libplanet.Explorer/Store/MySQLRichStore.cs
+++ b/Libplanet.Explorer/Store/MySQLRichStore.cs
@@ -1,3 +1,4 @@
+#nullable disable
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
diff --git a/Libplanet.Explorer/Store/MySQLRichStoreOptions.cs b/Libplanet.Explorer/Store/MySQLRichStoreOptions.cs
index 44b57a92d4b..44edab2fe83 100644
--- a/Libplanet.Explorer/Store/MySQLRichStoreOptions.cs
+++ b/Libplanet.Explorer/Store/MySQLRichStoreOptions.cs
@@ -1,3 +1,4 @@
+#nullable disable
 namespace Libplanet.Explorer.Store
 {
     public readonly struct MySQLRichStoreOptions