diff --git a/changelog.d/10827.bugfix b/changelog.d/10827.bugfix
new file mode 100644
index 0000000000..11a618bf82
--- /dev/null
+++ b/changelog.d/10827.bugfix
@@ -0,0 +1 @@
+Fix error in deprecated `/initialSync` endpoint when using the undocumented `from` and `to` parameters.
diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py
index 959f13de47..9a3b6f4acf 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -39,6 +39,8 @@ import logging
 from collections import namedtuple
 from typing import TYPE_CHECKING, Collection, Dict, List, Optional, Set, Tuple
 
+from frozendict import frozendict
+
 from twisted.internet import defer
 
 from synapse.api.filtering import Filter
@@ -379,7 +381,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore, metaclass=abc.ABCMeta):
                 if p > min_pos
             }
 
-        return RoomStreamToken(None, min_pos, positions)
+        return RoomStreamToken(None, min_pos, frozendict(positions))
 
     async def get_room_events_stream_for_rooms(
         self,
diff --git a/synapse/types.py b/synapse/types.py
index 90168ce8fa..ed831a5c1d 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -30,6 +30,7 @@ from typing import (
 )
 
 import attr
+from frozendict import frozendict
 from signedjson.key import decode_verify_key_bytes
 from unpaddedbase64 import decode_base64
 from zope.interface import Interface
@@ -457,6 +458,9 @@ class RoomStreamToken:
 
     Note: The `RoomStreamToken` cannot have both a topological part and an
     instance map.
+
+    For caching purposes, `RoomStreamToken`s and by extension, all their
+    attributes, must be hashable.
     """
 
     topological = attr.ib(
@@ -466,12 +470,12 @@ class RoomStreamToken:
     stream = attr.ib(type=int, validator=attr.validators.instance_of(int))
 
     instance_map = attr.ib(
-        type=Dict[str, int],
-        factory=dict,
+        type="frozendict[str, int]",
+        factory=frozendict,
         validator=attr.validators.deep_mapping(
             key_validator=attr.validators.instance_of(str),
             value_validator=attr.validators.instance_of(int),
-            mapping_validator=attr.validators.instance_of(dict),
+            mapping_validator=attr.validators.instance_of(frozendict),
         ),
     )
 
@@ -507,7 +511,7 @@ class RoomStreamToken:
                 return cls(
                     topological=None,
                     stream=stream,
-                    instance_map=instance_map,
+                    instance_map=frozendict(instance_map),
                 )
         except Exception:
             pass
@@ -540,7 +544,7 @@ class RoomStreamToken:
             for instance in set(self.instance_map).union(other.instance_map)
         }
 
-        return RoomStreamToken(None, max_stream, instance_map)
+        return RoomStreamToken(None, max_stream, frozendict(instance_map))
 
     def as_historical_tuple(self) -> Tuple[int, int]:
         """Returns a tuple of `(topological, stream)` for historical tokens.
@@ -593,6 +597,12 @@ class RoomStreamToken:
 
 @attr.s(slots=True, frozen=True)
 class StreamToken:
+    """A collection of positions within multiple streams.
+
+    For caching purposes, `StreamToken`s and by extension, all their attributes,
+    must be hashable.
+    """
+
     room_key = attr.ib(
         type=RoomStreamToken, validator=attr.validators.instance_of(RoomStreamToken)
     )