diff --git a/ContentGrabbingJi-CacheStore-local/src/main/java/net/lamgc/cgj/bot/cache/local/HashMapCacheStore.java b/ContentGrabbingJi-CacheStore-local/src/main/java/net/lamgc/cgj/bot/cache/local/HashMapCacheStore.java
index 8de517b..1c10794 100644
--- a/ContentGrabbingJi-CacheStore-local/src/main/java/net/lamgc/cgj/bot/cache/local/HashMapCacheStore.java
+++ b/ContentGrabbingJi-CacheStore-local/src/main/java/net/lamgc/cgj/bot/cache/local/HashMapCacheStore.java
@@ -38,7 +38,7 @@ public class HashMapCacheStore<V> extends HashCacheStore<Map<String, V>> impleme
 
     @Override
     public Set<String> mapFieldSet(String key) {
-        return getMap(key, false, Map::keySet, null);
+        return getMap(key, false, map -> Collections.unmodifiableSet(map.keySet()), null);
     }
 
     @Override