diff --git a/tests/unit/test_monkey_document.py b/tests/unit/test_monkey_document.py
index 73dfb04..d666f72 100644
--- a/tests/unit/test_monkey_document.py
+++ b/tests/unit/test_monkey_document.py
@@ -37,9 +37,20 @@ def _parse(pages=None, **kwargs):
     return parse_document_package(_package(pages, **kwargs))
 
 
+def _assert_rejected_on_purpose(exc: ResultInvalidError) -> None:
+    """拒绝必须出自显式校验,不能是 Python 自己绊倒后被外层边界吸收。
+
+    外层边界会把 TypeError/KeyError 也归为 ResultInvalidError,所以只断言异常类型,
+    「显式校验」与「碰巧在别处失败」无法区分(变异检查证实过: 删掉校验后测试仍绿)。
+    原因链里出现 TypeError/KeyError 即说明有校验缺口。
+    """
+    assert not isinstance(exc.__cause__, TypeError | KeyError), repr(exc.__cause__)
+
+
 def _invalid(pages=None, **kwargs):
-    with pytest.raises(ResultInvalidError):
+    with pytest.raises(ResultInvalidError) as ei:
         _parse(pages, **kwargs)
+    _assert_rejected_on_purpose(ei.value)
 
 
 class TestHappyPath:
@@ -195,6 +206,22 @@ class TestTableAndImage:
         assert elements[1].bbox == (1.0, 2.0, 3.0, 4.0)
         assert all(e.page_index == 0 for e in elements)
 
+    @pytest.mark.parametrize(
+        ("blocks", "reason"),
+        [
+            pytest.param("not-a-list", "blocks 无效", id="blocks-not-list"),
+            pytest.param(["not-a-dict"], "块结构无效", id="sub-block-not-dict"),
+            pytest.param([{"bbox": list(_BBOX)}], "块缺少 type", id="sub-block-without-type"),
+            pytest.param([{"type": "  ", "bbox": list(_BBOX)}], "块缺少 type", id="blank-type"),
+        ],
+    )
+    def test_malformed_nested_blocks_are_rejected_with_their_own_reason(self, blocks, reason):
+        """外层边界会把 TypeError 等也归为 ResultInvalid,所以必须断言**原因**,否则"显式校验"
+        与"碰巧在别处失败"无法区分(变异检查发现仅断言类型时删掉校验测试仍绿)。"""
+        block = {"type": "table", "bbox": list(_BBOX), "blocks": blocks}
+        with pytest.raises(ResultInvalidError, match=reason):
+            _parse([_page([block])])
+
     def test_empty_non_body_sub_block_is_not_emitted(self):
         caption = _body("table_caption", [_span("")])
         elements, _, _ = _parse([_page([_table_block(extra_subs=[caption])])])