diff --git a/changelog.d/14981.misc b/changelog.d/14981.misc
new file mode 100644
index 000000000..68ac8335f
--- /dev/null
+++ b/changelog.d/14981.misc
@@ -0,0 +1 @@
+Add tests for `_flatten_dict`.
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index 9bf92b976..20369f3df 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -473,6 +473,29 @@ def _flatten_dict(
     prefix: Optional[List[str]] = None,
     result: Optional[Dict[str, str]] = None,
 ) -> Dict[str, str]:
+    """
+    Given a JSON dictionary (or event) which might contain sub dictionaries,
+    flatten it into a single layer dictionary by combining the keys & sub-keys.
+
+    Any (non-dictionary), non-string value is dropped.
+
+    Transforms:
+
+        {"foo": {"bar": "test"}}
+
+    To:
+
+        {"foo.bar": "test"}
+
+    Args:
+        d: The event or content to continue flattening.
+        room_version: The room version object.
+        prefix: The key prefix (from outer dictionaries).
+        result: The result to mutate.
+
+    Returns:
+        The resulting dictionary.
+    """
     if prefix is None:
         prefix = []
     if result is None:
diff --git a/tests/push/test_push_rule_evaluator.py b/tests/push/test_push_rule_evaluator.py
index 81661e181..7c430c4ec 100644
--- a/tests/push/test_push_rule_evaluator.py
+++ b/tests/push/test_push_rule_evaluator.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Dict, List, Optional, Set, Union, cast
+from typing import Any, Dict, List, Optional, Set, Union, cast
 
 import frozendict
 
@@ -37,6 +37,30 @@ from tests import unittest
 from tests.test_utils.event_injection import create_event, inject_member_event
 
 
+class FlattenDictTestCase(unittest.TestCase):
+    def test_simple(self) -> None:
+        """Test a dictionary that isn't modified."""
+        input = {"foo": "abc"}
+        self.assertEqual(input, _flatten_dict(input))
+
+    def test_nested(self) -> None:
+        """Nested dictionaries become dotted paths."""
+        input = {"foo": {"bar": "abc"}}
+        self.assertEqual({"foo.bar": "abc"}, _flatten_dict(input))
+
+    def test_non_string(self) -> None:
+        """Non-string items are dropped."""
+        input: Dict[str, Any] = {
+            "woo": "woo",
+            "foo": True,
+            "bar": 1,
+            "baz": None,
+            "fuzz": [],
+            "boo": {},
+        }
+        self.assertEqual({"woo": "woo"}, _flatten_dict(input))
+
+
 class PushRuleEvaluatorTestCase(unittest.TestCase):
     def _get_evaluator(
         self,