diff --git a/src/common/algorithm/ReservoirSampling.h b/src/common/algorithm/ReservoirSampling.h
index 7ad90edcbad..5e9d1e3e5b5 100644
--- a/src/common/algorithm/ReservoirSampling.h
+++ b/src/common/algorithm/ReservoirSampling.h
@@ -33,7 +33,7 @@ class ReservoirSampling final {
       ++cnt_;
       return true;
     } else {
-      auto index = folly::Random::rand64(cnt_);
+      auto index = folly::Random::rand64(cnt_ + 1);
       if (index < num_) {
         samples_[index] = (std::move(sample));
         ++cnt_;
diff --git a/src/common/algorithm/test/ReservoirSamplingTest.cpp b/src/common/algorithm/test/ReservoirSamplingTest.cpp
index 0486b473ea7..57b153f1e9d 100644
--- a/src/common/algorithm/test/ReservoirSamplingTest.cpp
+++ b/src/common/algorithm/test/ReservoirSamplingTest.cpp
@@ -39,6 +39,21 @@ TEST(ReservoirSamplingTest, Sample) {
       EXPECT_EQ(2, result[2]);
     }
   }
+  {
+    std::unordered_set<int64_t> hit;
+    for (size_t time = 0; time < 1024; time++) {
+      ReservoirSampling<int64_t> sampler(1);
+      sampler.sampling(1);
+      sampler.sampling(2);
+      auto result = sampler.samples();
+      EXPECT_EQ(1, result.size());
+      EXPECT_TRUE(result[0] == 1 || result[0] == 2);
+      hit.insert(result[0]);
+    }
+    EXPECT_EQ(2, hit.size());
+    EXPECT_TRUE(hit.find(1) != hit.end());
+    EXPECT_TRUE(hit.find(2) != hit.end());
+  }
 }
 }  // namespace algorithm
 }  // namespace nebula