diff --git a/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/Storage.java b/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/Storage.java
index af1d000..8f6d19f 100644
--- a/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/Storage.java
+++ b/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/Storage.java
@@ -1,9 +1,12 @@
 package io.github.zrdzn.minecraft.lovelydrop.storage;
 
+import org.jetbrains.annotations.Nullable;
+
 import javax.sql.DataSource;
 
 /**
  * Class that represents storage by holding data source instance with associated storage type.
+ * Some storage types may not have data source instance, such as in-memory storage, therefore data source instance is nullable.
  */
 public class Storage {
 
@@ -15,6 +18,7 @@ public Storage(DataSource dataSource, StorageType type) {
         this.type = type;
     }
 
+    @Nullable
     public DataSource getDataSource() {
         return this.dataSource;
     }
diff --git a/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/StorageFactory.java b/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/StorageFactory.java
index 9e899d1..4d99761 100644
--- a/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/StorageFactory.java
+++ b/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/storage/StorageFactory.java
@@ -122,6 +122,12 @@ public Storage createStorage() {
 
                 break;
             case IN_MEMORY:
+                logger.info("Choosing in-memory as a storage provider.");
+
+                dataSource = null;
+                storageType = StorageType.IN_MEMORY;
+
+                break;
             default:
                 throw new IllegalArgumentException("There is no such storage type.");
         }
diff --git a/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/user/infra/SqlUserSettingRepository.java b/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/user/infra/SqlUserSettingRepository.java
index 0596cd5..2bef47a 100644
--- a/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/user/infra/SqlUserSettingRepository.java
+++ b/plugin/src/main/java/io/github/zrdzn/minecraft/lovelydrop/user/infra/SqlUserSettingRepository.java
@@ -4,13 +4,7 @@
 import java.sql.PreparedStatement;
 import java.sql.ResultSet;
 import java.sql.SQLException;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Map;
-import java.util.Optional;
-import java.util.Set;
-import java.util.UUID;
+import java.util.*;
 import javax.sql.DataSource;
 import com.google.gson.Gson;
 import com.google.gson.reflect.TypeToken;
@@ -31,8 +25,8 @@ class SqlUserSettingRepository implements UserSettingRepository {
 
     public SqlUserSettingRepository(DataSource dataSource, Gson gson, String createOrUpdateUserSettingQuery,
                                     String findUserSettingByPlayerId) {
-        this.dataSource = dataSource;
-        this.gson = gson;
+        this.dataSource = Objects.requireNonNull(dataSource, "Data source cannot be null.");
+        this.gson = Objects.requireNonNull(gson, "Gson cannot be null.");
 
         this.createOrUpdateUserSettingQuery = createOrUpdateUserSettingQuery;
         this.findUserSettingByPlayerId = findUserSettingByPlayerId;