diff --git a/src/Libplanet.Common/FixedSizedQueue.cs b/src/Libplanet.Common/FixedSizedQueue.cs
index 16c993a7ea9..9b7e5ae1253 100644
--- a/src/Libplanet.Common/FixedSizedQueue.cs
+++ b/src/Libplanet.Common/FixedSizedQueue.cs
@@ -46,7 +46,7 @@ public FixedSizedQueue(int size)
                 while (Count > Size)
                 {
                     // Ensure we don't exceed the maximum size.
-                    TryDequeue(out T _);
+                    TryDequeue(out T? _);
                 }
             }
         }
diff --git a/src/Libplanet.Store/BlockSet.cs b/src/Libplanet.Store/BlockSet.cs
index 59b45b83673..e67c4644743 100644
--- a/src/Libplanet.Store/BlockSet.cs
+++ b/src/Libplanet.Store/BlockSet.cs
@@ -4,7 +4,6 @@
 using System.Linq;
 using BitFaster.Caching;
 using BitFaster.Caching.Lru;
-using Caching;
 using Libplanet.Types.Blocks;
 
 namespace Libplanet.Store
@@ -157,7 +156,7 @@ public IEnumerator<KeyValuePair<BlockHash, Block>> GetEnumerator()
 
         private Block? GetBlock(BlockHash key)
         {
-            if (_cache.TryGet(key, out Block cached))
+            if (_cache.TryGet(key, out Block? cached))
             {
                 if (_store.ContainsBlock(key))
                 {