diff --git a/msgspec/_core.c b/msgspec/_core.c
index 147ad95a..f70d213d 100644
--- a/msgspec/_core.c
+++ b/msgspec/_core.c
@@ -8342,6 +8342,9 @@ TypedDictInfo_error_missing(TypedDictInfo *self, PyObject *dict, PathNode *path)
             }
         }
     }
+    // Should be unreachable, but may happen if the TypedDict info
+    // is inconsistent.
+    assert(0);
 }
 
 static int
diff --git a/msgspec/_utils.py b/msgspec/_utils.py
index 6d338104..ca7815ff 100644
--- a/msgspec/_utils.py
+++ b/msgspec/_utils.py
@@ -227,6 +227,13 @@ def get_typeddict_info(obj):
             hints[k] = v.__args__[0]
         else:
             hints[k] = v
+
+    # This can happen if there is a bug in the TypedDict implementation;
+    # such a bug was present in Python 3.14.
+    if not all(k in hints for k in required):
+        raise RuntimeError(
+            f"Required set {required} contains keys that are no in hints: {hints.keys()}"
+        )
     return hints, required
 
 
diff --git a/tests/test_common.py b/tests/test_common.py
index 38898be3..30f47be0 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -2112,6 +2112,24 @@ class Ex(Base, total=False):
             dec.decode(proto.encode({"b": "two"}))
         assert "Object missing required field `a`" == str(rec.value)
 
+    @pytest.mark.parametrize("use_typing_extensions", [False, True])
+    def test_broken_typeddict(self, proto, use_typing_extensions):
+        # Check that we don't crash if a TypedDict has incorrect
+        # introspection data.
+        if use_typing_extensions:
+            tex = pytest.importorskip("typing_extensions")
+            cls = tex.TypedDict
+        else:
+            cls = TypedDict
+
+        class Ex(cls, total=False):
+            c: str
+        Ex.__annotations__ = {"c": "str"}
+        Ex.__required_keys__ = {"a", "b"}
+
+        with pytest.raises(RuntimeError):
+            proto.Decoder(Ex)
+
     @pytest.mark.parametrize("use_typing_extensions", [False, True])
     def test_required_and_notrequired(self, proto, use_typing_extensions):
         if use_typing_extensions: