diff --git a/src/types/redis_hyperloglog.cc b/src/types/redis_hyperloglog.cc
index 63ee0b2b926..952177265f0 100644
--- a/src/types/redis_hyperloglog.cc
+++ b/src/types/redis_hyperloglog.cc
@@ -240,7 +240,7 @@ rocksdb::Status HyperLogLog::Merge(engine::Context &ctx, const Slice &dest_user_
   std::vector<std::string> registers;
   HyperLogLogMetadata metadata;
 
-  rocksdb::Status s = GetMetadata(ctx, dest_user_key, &metadata);
+  rocksdb::Status s = GetMetadata(ctx, dest_key, &metadata);
   if (!s.ok() && !s.IsNotFound()) return s;
   {
     std::vector<Slice> all_user_keys;
diff --git a/tests/gocase/unit/hyperloglog/hyperloglog_test.go b/tests/gocase/unit/hyperloglog/hyperloglog_test.go
index 62fdff40d66..28fc949cf89 100644
--- a/tests/gocase/unit/hyperloglog/hyperloglog_test.go
+++ b/tests/gocase/unit/hyperloglog/hyperloglog_test.go
@@ -23,6 +23,7 @@ import (
 	"context"
 	"fmt"
 	"testing"
+	"time"
 
 	"github.com/apache/kvrocks/tests/gocase/util"
 	"github.com/stretchr/testify/require"
@@ -213,4 +214,56 @@ func TestHyperLogLog(t *testing.T) {
 		require.NoError(t, err)
 		require.EqualValues(t, 10, card)
 	})
+
+	t.Run("PFMERGE into existing dest preserves dest data", func(t *testing.T) {
+		require.NoError(t, rdb.Do(ctx, "DEL", "dest", "src1", "src2").Err())
+
+		_, err := rdb.PFAdd(ctx, "dest", "a", "b", "c").Result()
+		require.NoError(t, err)
+		_, err = rdb.PFAdd(ctx, "src1", "d", "e").Result()
+		require.NoError(t, err)
+		_, err = rdb.PFAdd(ctx, "src2", "f").Result()
+		require.NoError(t, err)
+
+		// Merge src1 and src2 into existing dest
+		_, err = rdb.PFMerge(ctx, "dest", "src1", "src2").Result()
+		require.NoError(t, err)
+
+		// dest should have all 6 elements
+		card, err := rdb.PFCount(ctx, "dest").Result()
+		require.NoError(t, err)
+		require.EqualValues(t, 6, card)
+
+		// Merge again into dest to verify idempotency
+		_, err = rdb.PFMerge(ctx, "dest", "src1").Result()
+		require.NoError(t, err)
+		card, err = rdb.PFCount(ctx, "dest").Result()
+		require.NoError(t, err)
+		require.EqualValues(t, 6, card)
+	})
+
+	// PFMERGE must load dest metadata using the namespace-prefixed key (same as storage).
+	// If GetMetadata used the raw user key, it would miss the record, keep default Metadata (expire=0),
+	// and the following PFMERGE would rewrite metadata without preserving TTL (TTL becomes "no expiry").
+	t.Run("PFMERGE into existing dest preserves TTL", func(t *testing.T) {
+		require.NoError(t, rdb.Do(ctx, "DEL", "dest_ttl", "src_ttl").Err())
+
+		_, err := rdb.PFAdd(ctx, "dest_ttl", "a", "b").Result()
+		require.NoError(t, err)
+		ok, err := rdb.Expire(ctx, "dest_ttl", 3600*time.Second).Result()
+		require.NoError(t, err)
+		require.True(t, ok)
+
+		ttlBefore := rdb.TTL(ctx, "dest_ttl").Val()
+		require.Greater(t, ttlBefore, time.Duration(0))
+
+		_, err = rdb.PFAdd(ctx, "src_ttl", "c").Result()
+		require.NoError(t, err)
+		_, err = rdb.PFMerge(ctx, "dest_ttl", "src_ttl").Result()
+		require.NoError(t, err)
+
+		ttlAfter := rdb.TTL(ctx, "dest_ttl").Val()
+		require.Greater(t, ttlAfter, time.Duration(0),
+			"PFMERGE must keep dest TTL when dest already exists (metadata must be read with ns-prefixed key)")
+	})
 }