diff --git a/changelog.d/10606.bugfix b/changelog.d/10606.bugfix
new file mode 100644
index 000000000000..bab9fd2a612a
--- /dev/null
+++ b/changelog.d/10606.bugfix
@@ -0,0 +1 @@
+Fix errors on /sync when read receipt data is a string. Only affects homeservers with the experimental flag for [MSC2285](https://github.com/matrix-org/matrix-doc/pull/2285) enabled. Contributed by @SimonBrandner.
diff --git a/synapse/handlers/receipts.py b/synapse/handlers/receipts.py
index 5fd45257004c..fb495229a7e1 100644
--- a/synapse/handlers/receipts.py
+++ b/synapse/handlers/receipts.py
@@ -188,7 +188,14 @@ def filter_out_hidden(events: List[JsonDict], user_id: str) -> List[JsonDict]:
 
                 new_users = {}
                 for rr_user_id, user_rr in m_read.items():
-                    hidden = user_rr.get("hidden", None)
+                    try:
+                        hidden = user_rr.get("hidden")
+                    except AttributeError:
+                        # Due to https://github.com/matrix-org/synapse/issues/10376
+                        # there are cases where user_rr is a string, in those cases
+                        # we just ignore the read receipt
+                        continue
+
                     if hidden is not True or rr_user_id == user_id:
                         new_users[rr_user_id] = user_rr.copy()
                         # If hidden has a value replace hidden with the correct prefixed key
diff --git a/tests/handlers/test_receipts.py b/tests/handlers/test_receipts.py
index 93a9a084b24b..732a12c9bd08 100644
--- a/tests/handlers/test_receipts.py
+++ b/tests/handlers/test_receipts.py
@@ -286,6 +286,29 @@ def test_filters_out_receipt_event_with_only_hidden_receipt_and_ignores_rest(sel
             ],
         )
 
+    def test_handles_string_data(self):
+        """
+        Tests that an invalid shape for read-receipts is handled.
+        Context: https://github.com/matrix-org/synapse/issues/10603
+        """
+
+        self._test_filters_hidden(
+            [
+                {
+                    "content": {
+                        "$14356419edgd14394fHBLK:matrix.org": {
+                            "m.read": {
+                                "@rikj:jki.re": "string",
+                            }
+                        },
+                    },
+                    "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+                    "type": "m.receipt",
+                },
+            ],
+            [],
+        )
+
     def _test_filters_hidden(
         self, events: List[JsonDict], expected_output: List[JsonDict]
     ):