diff --git a/include/flashinfer/attention/cascade.cuh b/include/flashinfer/attention/cascade.cuh
index a5ddd8d4..5445156b 100644
--- a/include/flashinfer/attention/cascade.cuh
+++ b/include/flashinfer/attention/cascade.cuh
@@ -157,9 +157,29 @@ __global__ void MergeStatesKernel(DTypeIn* __restrict__ V, float* __restrict__ S
   uint32_t tx = threadIdx.x, ty = threadIdx.y;
   uint32_t pos = blockIdx.x;
   uint32_t head_idx = ty;
-  state_t<vec_size> st;
+
+  if (num_index_sets == 0) {
+    vec_t<DTypeOut, vec_size> v;
+    v.fill(DTypeOut(0));
+    v.store(v_merged + (pos * num_heads + head_idx) * head_dim + tx * vec_size);
+    if (s_merged != nullptr) {
+      s_merged[pos * num_heads + head_idx] = -5e4;
+    }
+    return;
+  }
+
+  if (num_index_sets == 1) {
+    vec_t<DTypeOut, vec_size> v;
+    v.cast_load(V + (pos * num_heads + head_idx) * head_dim + tx * vec_size);
+    v.store(v_merged + (pos * num_heads + head_idx) * head_dim + tx * vec_size);
+    if (s_merged != nullptr) {
+      s_merged[pos * num_heads + head_idx] = S[pos * num_heads + head_idx];
+    }
+    return;
+  }
 
   vec_t<float, vec_size> v_merged_vec;
+  state_t<vec_size> st;
   v_merged_vec.fill(0.f);
 #pragma unroll 2
   for (uint32_t iter = 0; iter < num_index_sets; ++iter) {
@@ -296,6 +316,25 @@ __global__ void VariableLengthMergeStatesKernel(DTypeIn* __restrict__ V, float*
   float* s_smem = (float*)(smem + num_smem_stages * bdy * head_dim * sizeof(DTypeIn));
   const uint32_t num_index_sets = indptr[pos + 1] - indptr[pos];
 
+  if (num_index_sets == 0) {
+    vec_t<DTypeOut, vec_size> v;
+    v.fill(DTypeOut(0));
+    v.store(v_merged + (pos * num_heads + head_idx) * head_dim + tx * vec_size);
+    if (s_merged != nullptr) {
+      s_merged[pos * num_heads + head_idx] = -5e4;
+    }
+    return;
+  }
+
+  if (num_index_sets == 1) {
+    vec_t<DTypeOut, vec_size> v;
+    v.cast_load(V + (indptr[pos] * num_heads + head_idx) * head_dim + tx * vec_size);
+    v.store(v_merged + (pos * num_heads + head_idx) * head_dim + tx * vec_size);
+    if (s_merged != nullptr) {
+      s_merged[pos * num_heads + head_idx] = S[indptr[pos] * num_heads + head_idx];
+    }
+  }
+
 #pragma unroll
   for (uint32_t iter = 0; iter < num_smem_stages; ++iter) {
     cp_async::pred_load<vec_bits, PrefetchMode::kPrefetch, SharedMemFillMode::kNoFill>(
diff --git a/src/test_cascade.cu b/src/test_cascade.cu
index 4bbce95a..5d7d28cd 100644
--- a/src/test_cascade.cu
+++ b/src/test_cascade.cu
@@ -136,7 +136,6 @@ void _TestVariableLengthMergeKernelCorrectness(size_t seq_len, size_t num_heads,
 template <typename T>
 void _TestMergeKernelCorrectness(size_t num_index_sets, size_t seq_len, size_t num_heads,
                                  size_t head_dim, bool sparse_s) {
-  EXPECT_GT(num_index_sets, 1) << "num_index_sets must be greater than 1";
   std::vector<T> V_host(seq_len * num_index_sets * num_heads * head_dim);
   std::vector<float> V_host_trans_f32(num_index_sets * seq_len * num_heads * head_dim);
   std::vector<float> S_host(seq_len * num_index_sets * num_heads);
@@ -178,20 +177,25 @@ void _TestMergeKernelCorrectness(size_t num_index_sets, size_t seq_len, size_t n
   thrust::device_vector<T> V_merged_1_device(seq_len * num_heads * head_dim);
   thrust::device_vector<float> S_merged_1_device(seq_len * num_heads);
 
-  // Method 0: use MergeState
-  MergeState(thrust::raw_pointer_cast(V_device_trans_f32.data()),
-             thrust::raw_pointer_cast(S_device_trans.data()),
-             thrust::raw_pointer_cast(V_device_trans_f32.data() + seq_len * num_heads * head_dim),
-             thrust::raw_pointer_cast(S_device_trans.data() + seq_len * num_heads),
-             thrust::raw_pointer_cast(V_merged_0_device.data()),
-             thrust::raw_pointer_cast(S_merged_0_device.data()), seq_len, num_heads, head_dim);
-  for (uint i = 2; i < num_index_sets; ++i) {
-    MergeStateInPlace(
-        thrust::raw_pointer_cast(V_merged_0_device.data()),
-        thrust::raw_pointer_cast(S_merged_0_device.data()),
-        thrust::raw_pointer_cast(V_device_trans_f32.data() + i * seq_len * num_heads * head_dim),
-        thrust::raw_pointer_cast(S_device_trans.data() + i * seq_len * num_heads), seq_len,
-        num_heads, head_dim);
+  if (num_index_sets > 1) {
+    // Method 0: use MergeState
+    MergeState(thrust::raw_pointer_cast(V_device_trans_f32.data()),
+               thrust::raw_pointer_cast(S_device_trans.data()),
+               thrust::raw_pointer_cast(V_device_trans_f32.data() + seq_len * num_heads * head_dim),
+               thrust::raw_pointer_cast(S_device_trans.data() + seq_len * num_heads),
+               thrust::raw_pointer_cast(V_merged_0_device.data()),
+               thrust::raw_pointer_cast(S_merged_0_device.data()), seq_len, num_heads, head_dim);
+    for (uint i = 2; i < num_index_sets; ++i) {
+      MergeStateInPlace(
+          thrust::raw_pointer_cast(V_merged_0_device.data()),
+          thrust::raw_pointer_cast(S_merged_0_device.data()),
+          thrust::raw_pointer_cast(V_device_trans_f32.data() + i * seq_len * num_heads * head_dim),
+          thrust::raw_pointer_cast(S_device_trans.data() + i * seq_len * num_heads), seq_len,
+          num_heads, head_dim);
+    }
+  } else {
+    V_merged_0_device = V_device;
+    S_merged_0_device = S_device;
   }
 
   // Method 1: use MergeStates
@@ -479,7 +483,7 @@ void _TestTwoLevelSinglePrefixCascadeAppendCorrectness(size_t batch_size,
 
 template <typename T>
 void TestMergeKernelCorrectness() {
-  for (size_t num_index_sets : {2, 9, 81, 513}) {
+  for (size_t num_index_sets : {1, 2, 9, 81, 513}) {
     for (size_t seq_len : {4, 16, 77}) {
       for (size_t num_heads : {1, 21, 32}) {
         for (size_t head_dim : {64, 128, 256}) {