diff --git a/tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py b/tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py
index 3a7677e7511f..b7e500fe6d7f 100644
--- a/tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py
+++ b/tests/v1/attention/test_flashinfer_sparse_mla_sm120_api.py
@@ -4,6 +4,7 @@
 
 from types import SimpleNamespace
 
+import pytest
 import torch
 
 from vllm.config import set_current_vllm_config
@@ -12,13 +13,23 @@
 from vllm.v1.attention.backends.mla.flashinfer_mla_sparse import (
     FlashInferMLASparseSM120Backend,
 )
+from vllm.v1.attention.backends.mla.flashinfer_mla_sparse_sm120 import (
+    FlashInferMLASparseSM120Impl,
+)
 from vllm.v1.attention.backends.registry import AttentionBackendEnum
 
 
-def _fake_vllm_config(model_type: str) -> SimpleNamespace:
+def _fake_vllm_config(
+    model_type: str,
+    *,
+    index_topk: int | None = 2048,
+) -> SimpleNamespace:
+    hf_text_config = SimpleNamespace(model_type=model_type)
+    if index_topk is not None:
+        hf_text_config.index_topk = index_topk
     return SimpleNamespace(
         model_config=SimpleNamespace(
-            hf_text_config=SimpleNamespace(model_type=model_type, index_topk=2048),
+            hf_text_config=hf_text_config,
         ),
     )
 
@@ -52,3 +63,79 @@ def test_v32_glm_sm120_backend_accepts_glm_block_size(
         )
 
     assert invalid_reasons == []
+
+
+def test_sm120_sparse_backend_rejects_model_without_index_topk(
+    monkeypatch,
+) -> None:
+    monkeypatch.setattr(fi_utils, "has_flashinfer_sparse_mla_sm120", lambda: True)
+
+    with set_current_vllm_config(_fake_vllm_config("glm4_moe", index_topk=None)):
+        invalid_reasons = FlashInferMLASparseSM120Backend.validate_configuration(
+            head_size=576,
+            dtype=torch.bfloat16,
+            kv_cache_dtype="fp8",
+            block_size=256,
+            use_mla=True,
+            has_sink=False,
+            use_sparse=True,
+            use_mm_prefix=False,
+            use_per_head_quant_scales=False,
+            device_capability=DeviceCapability(12, 0),
+            attn_type="decoder",
+        )
+
+    assert invalid_reasons == [
+        "FLASHINFER_MLA_SPARSE_SM120 requires a model with index_topk config"
+    ]
+
+
+def test_sm120_sparse_impl_accepts_shared_topk_buffer_without_indexer(
+    monkeypatch,
+) -> None:
+    monkeypatch.setattr(fi_utils, "has_flashinfer_sparse_mla_sm120", lambda: True)
+    topk_indices_buffer = torch.empty(4, 2048, dtype=torch.int32)
+
+    with set_current_vllm_config(_fake_vllm_config("glm4_moe")):
+        impl = FlashInferMLASparseSM120Impl(
+            num_heads=16,
+            head_size=576,
+            scale=1.0,
+            num_kv_heads=1,
+            alibi_slopes=None,
+            sliding_window=None,
+            kv_cache_dtype="fp8_ds_mla",
+            logits_soft_cap=None,
+            attn_type="decoder",
+            kv_sharing_target_layer_name=None,
+            indexer=None,
+            kv_lora_rank=512,
+            qk_nope_head_dim=512,
+            qk_rope_head_dim=64,
+            topk_indices_buffer=topk_indices_buffer,
+        )
+
+    assert impl.topk_indices_buffer is topk_indices_buffer
+
+
+def test_sm120_sparse_impl_rejects_missing_topk_indices() -> None:
+    with (
+        set_current_vllm_config(_fake_vllm_config("glm4_moe")),
+        pytest.raises(ValueError, match="requires sparse-MLA top-k indices"),
+    ):
+        FlashInferMLASparseSM120Impl(
+            num_heads=16,
+            head_size=576,
+            scale=1.0,
+            num_kv_heads=1,
+            alibi_slopes=None,
+            sliding_window=None,
+            kv_cache_dtype="fp8_ds_mla",
+            logits_soft_cap=None,
+            attn_type="decoder",
+            kv_sharing_target_layer_name=None,
+            indexer=None,
+            kv_lora_rank=512,
+            qk_nope_head_dim=512,
+            qk_rope_head_dim=64,
+        )
diff --git a/vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py b/vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py
index d802f568836a..e4c62eb37438 100644
--- a/vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py
+++ b/vllm/v1/attention/backends/mla/flashinfer_mla_sparse_sm120.py
@@ -88,6 +88,12 @@ def __init__(
             if indexer is not None
             else mla_args.get("topk_indices_buffer")
         )
+        if self.topk_indices_buffer is None:
+            raise ValueError(
+                "FLASHINFER_MLA_SPARSE_SM120 requires sparse-MLA top-k "
+                "indices from an indexer or shared topk_indices_buffer "
+                "(model with index_topk in its config)."
+            )
         from vllm.utils.flashinfer import has_flashinfer_sparse_mla_sm120
 
         if not has_flashinfer_sparse_mla_sm120():
@@ -95,8 +101,6 @@ def __init__(
                 "FLASHINFER_MLA_SPARSE_SM120 requires FlashInfer's "
                 "sparse MLA decode API."
             )
-        assert self.topk_indices_buffer is not None
-
         self.supports_quant_query_input = False
         self._workspace_buffer: torch.Tensor | None = None