diff --git a/src/flask/mongo/repository.py b/src/flask/mongo/repository.py
index 0044244..5aceb89 100644
--- a/src/flask/mongo/repository.py
+++ b/src/flask/mongo/repository.py
@@ -230,7 +230,7 @@ class BeerRepository:
         generated or is a note explaining that it could not be.
         """
         doc_id = _as_doc_id(reading_id)
-        if doc_id is None:
+        if not doc_id:
             return 0
         # `$in` matches either form.  A plain list would be an exact
         # ARRAY match in Mongo, which is not what is wanted here.
@@ -422,7 +422,11 @@ def _as_doc_id(value: object):
         except Exception:
             # Not hex - it can only be the original app's string id.
             return [value]
-    return None
+    # Anything else - a dict, a list, an int, None - is not a reading
+    # id.  An EMPTY LIST is returned rather than None: it is a valid
+    # `$in` argument that matches nothing, so the helper cannot be
+    # misused into a query error or into matching every document.
+    return []
 
 
 def _as_date(value: object) -> str:
diff --git a/tests/test_brew_advice.py b/tests/test_brew_advice.py
index f67deae..7098057 100644
--- a/tests/test_brew_advice.py
+++ b/tests/test_brew_advice.py
@@ -414,6 +414,14 @@ def test_the_reading_id_cannot_be_used_to_inject_a_query(repo):
         modified = repo.set_reading_advice(hostile, "INJECTED", True)
         assert modified == 0, f"{hostile!r} matched something"
 
+    # The helper itself must return something that is safe to put in a
+    # query - an empty list matches nothing.  Returning None would raise
+    # "$in needs an array" if it were ever used without a guard.
+    from mongo.repository import _as_doc_id
+
+    for hostile in [{"$ne": None}, {"$gt": ""}, ["a"], 123, None, ""]:
+        assert _as_doc_id(hostile) == [], f"{hostile!r} produced a usable id"
+
     # And no document was touched.
     for doc in repo.readings.find({}):
         assert doc.get("advice") is None