diff --git a/tests/kernels/moe/test_flashinfer.py b/tests/kernels/moe/test_flashinfer.py
index d524b5667..5ecef3dbd 100644
--- a/tests/kernels/moe/test_flashinfer.py
+++ b/tests/kernels/moe/test_flashinfer.py
@@ -398,3 +398,80 @@ def test_convert_moe_weights_to_flashinfer_trtllm_block_layout(
 
     assert w13_converted.shape[0] == num_experts
     assert w2_converted.shape[0] == num_experts
+
+
+def test_flashinfer_blockscale_fp8_none_expert_group(monkeypatch):
+    """Test that flashinfer_fused_moe_blockscale_fp8 handles num_expert_group=None.
+
+    Regression test for https://github.com/vllm-project/vllm/issues/34477
+    MiniMax-M2.1 uses sigmoid scoring with e_score_correction_bias but no
+    grouped top-k, resulting in num_expert_group=None. This triggered a crash
+    in the flashinfer kernel when DeepSeekV3 routing was selected.
+    """
+    if not current_platform.has_device_capability(100):
+        pytest.skip("Test requires SM >= 100 (Blackwell)")
+
+    import vllm.model_executor.layers.fused_moe.flashinfer_trtllm_moe  # noqa: E501, F401
+    from tests.kernels.quant_utils import native_per_token_group_quant_fp8
+
+    set_random_seed(7)
+    monkeypatch.setenv("VLLM_FUSED_MOE_CHUNK_SIZE", "8192")
+
+    e = 16  # num_experts (must be divisible by 4)
+    topk = 6  # top_k > 1 triggers DeepSeekV3 routing with sigmoid
+    m, n, k = 10, 4096, 5120
+    block_shape = [128, 128]
+    block_k = block_shape[1]
+
+    with set_current_vllm_config(vllm_config):
+        # Create BF16 hidden states
+        x = torch.randn((m, k), device="cuda", dtype=torch.bfloat16) / 10
+
+        # Create FP8 block-scale quantized weights
+        w13_bf16 = torch.randn((e, 2 * n, k), device="cuda", dtype=torch.bfloat16) / 10
+        w2_bf16 = torch.randn((e, k, n), device="cuda", dtype=torch.bfloat16) / 10
+
+        # Quantize weights per-block to FP8
+        w13_fp8_list, w13_scale_list = [], []
+        w2_fp8_list, w2_scale_list = [], []
+        for i in range(e):
+            wq, ws = native_per_token_group_quant_fp8(w13_bf16[i], block_k)
+            w13_fp8_list.append(wq)
+            w13_scale_list.append(ws)
+
+            wq, ws = native_per_token_group_quant_fp8(w2_bf16[i], block_k)
+            w2_fp8_list.append(wq)
+            w2_scale_list.append(ws)
+
+        w13_fp8 = torch.stack(w13_fp8_list)
+        w13_scale = torch.stack(w13_scale_list)
+        w2_fp8 = torch.stack(w2_fp8_list)
+        w2_scale = torch.stack(w2_scale_list)
+
+        # DeepSeekV3 routing uses float32 logits + optional bias
+        routing_logits = torch.randn((m, e), device="cuda", dtype=torch.float32)
+        routing_bias = torch.randn(e, device="cuda", dtype=torch.float32)
+
+        # This should NOT crash with num_expert_group=None
+        output = torch.ops.vllm.flashinfer_fused_moe_blockscale_fp8(
+            routing_logits=routing_logits,
+            routing_bias=routing_bias,
+            x=x,
+            w13_weight=w13_fp8,
+            w13_weight_scale_inv=w13_scale,
+            w2_weight=w2_fp8,
+            w2_weight_scale_inv=w2_scale,
+            global_num_experts=e,
+            top_k=topk,
+            num_expert_group=None,
+            topk_group=None,
+            intermediate_size=n,
+            expert_offset=0,
+            local_num_experts=e,
+            block_shape=block_shape,
+            routing_method_type=RoutingMethodType.DeepSeekV3,
+            routed_scaling=1.0,
+        )
+
+        assert output is not None
+        assert output.shape == (m, k)
diff --git a/vllm/model_executor/layers/fused_moe/flashinfer_trtllm_moe.py b/vllm/model_executor/layers/fused_moe/flashinfer_trtllm_moe.py
index b2d571dd8..d86896e54 100644
--- a/vllm/model_executor/layers/fused_moe/flashinfer_trtllm_moe.py
+++ b/vllm/model_executor/layers/fused_moe/flashinfer_trtllm_moe.py
@@ -201,6 +201,7 @@ def flashinfer_fused_moe_blockscale_fp8(
 ) -> torch.Tensor:
     from vllm.utils.flashinfer import flashinfer_trtllm_fp8_block_scale_moe
 
+    num_expert_group = num_expert_group if num_expert_group is not None else 0
     topk_group = topk_group if topk_group is not None else 0
     assert top_k <= global_num_experts
     assert top_k <= 10