diff --git a/README.rst b/README.rst
index af0c212..03baff3 100644
--- a/README.rst
+++ b/README.rst
@@ -15,7 +15,7 @@ Features
   U+0056, to keep the output as small as possible.
 * Uses the shortest escape sequence for each escaped character.
 * Encodes the JSON as UTF-8.
-* Can encode ``frozendict`` immutable dictionaries.
+* Can be configured to encode custom types unknown to the stdlib JSON encoder.
 
 Supports Python versions 3.7 and newer.
 
@@ -59,3 +59,20 @@ The underlying JSON implementation can be chosen with the following:
     which uses the standard library json module).
 
 .. _simplejson: https://simplejson.readthedocs.io/
+
+A preserialisation hook allows you to encode objects which aren't encodable by the
+standard library ``JSONEncoder``.
+
+.. code:: python
+
+    import canonicaljson
+    from typing import Dict
+
+    class CustomType:
+        pass
+
+    def callback(c: CustomType) -> Dict[str, str]:
+        return {"Hello": "world!"}
+
+    canonicaljson.register_preserialisation_callback(CustomType, callback)
+    assert canonicaljson.encode_canonical_json(CustomType()) == b'{"Hello":"world!"}'
diff --git a/setup.cfg b/setup.cfg
index 4b707de..60417f4 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -34,12 +34,6 @@ install_requires =
     typing_extensions>=4.0.0; python_version < '3.8'
 
 
-[options.extras_require]
-# frozendict support can be enabled using the `canonicaljson[frozendict]` syntax
-frozendict =
-    frozendict>=1.0
-
-
 [options.package_data]
 canonicaljson = py.typed
 
diff --git a/src/canonicaljson/__init__.py b/src/canonicaljson/__init__.py
index 2e33b66..24ed332 100644
--- a/src/canonicaljson/__init__.py
+++ b/src/canonicaljson/__init__.py
@@ -13,33 +13,56 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+import functools
 import platform
-from typing import Any, Generator, Iterator, Optional, Type
+from typing import Any, Callable, Generator, Iterator, Type, TypeVar
 
 try:
     from typing import Protocol
 except ImportError:  # pragma: no cover
     from typing_extensions import Protocol  # type: ignore[assignment]
 
-frozendict_type: Optional[Type[Any]]
-try:
-    from frozendict import frozendict as frozendict_type
-except ImportError:
-    frozendict_type = None  # pragma: no cover
 
 __version__ = "1.6.5"
 
 
-def _default(obj: object) -> object:  # pragma: no cover
-    if type(obj) is frozendict_type:
-        # If frozendict is available and used, cast `obj` into a dict
-        return dict(obj)  # type: ignore[call-overload]
+@functools.singledispatch
+def _preprocess_for_serialisation(obj: object) -> object:  # pragma: no cover
+    """Transform an `obj` into something the JSON library knows how to encode.
+
+    This is only called for types that the JSON library does not recognise.
+    """
     raise TypeError(
         "Object of type %s is not JSON serializable" % obj.__class__.__name__
     )
 
 
+T = TypeVar("T")
+
+
+def register_preserialisation_callback(
+    data_type: Type[T], callback: Callable[[T], object]
+) -> None:
+    """
+    Register a `callback` to preprocess `data_type` objects unknown to the JSON encoder.
+
+    When canonicaljson encodes an object `x` at runtime that its JSON library does not
+    know how to encode, it will
+      - select a `callback`,
+      - compute `y = callback(x)`, then
+      - JSON-encode `y` and return the result.
+
+    The `callback` should return an object that is JSON-serialisable by the stdlib
+    json module.
+
+    If this is called multiple times with the same `data_type`, the most recently
+    registered callback is used when serialising that `data_type`.
+    """
+    if data_type is object:
+        raise ValueError("Cannot register callback for the `object` type")
+    _preprocess_for_serialisation.register(data_type, callback)
+
+
 class Encoder(Protocol):  # pragma: no cover
     def encode(self, data: object) -> str:
         pass
@@ -77,7 +100,7 @@ def set_json_library(json_lib: JsonLibrary) -> None:
         allow_nan=False,
         separators=(",", ":"),
         sort_keys=True,
-        default=_default,
+        default=_preprocess_for_serialisation,
     )
 
     global _pretty_encoder
@@ -86,7 +109,7 @@ def set_json_library(json_lib: JsonLibrary) -> None:
         allow_nan=False,
         indent=4,
         sort_keys=True,
-        default=_default,
+        default=_preprocess_for_serialisation,
     )
 
 
diff --git a/tox.ini b/tox.ini
index a893107..63b9d58 100644
--- a/tox.ini
+++ b/tox.ini
@@ -33,7 +33,6 @@ commands = python -m black --check --diff src tests
 [testenv:mypy]
 deps =
     mypy==1.0
-    types-frozendict==2.0.8
     types-simplejson==3.17.5
     types-setuptools==57.4.14
 commands = mypy src tests

diff --git a/tests/test_canonicaljson.py b/tests/test_canonicaljson.py
index f1fac9a..44422d5 100644
--- a/tests/test_canonicaljson.py
+++ b/tests/test_canonicaljson.py
@@ -13,16 +13,17 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from unittest.mock import Mock
 
 from math import inf, nan
 
 from canonicaljson import (
     encode_canonical_json,
     encode_pretty_printed_json,
-    frozendict_type,
     iterencode_canonical_json,
     iterencode_pretty_printed_json,
     set_json_library,
+    register_preserialisation_callback,
 )
 
 import unittest
@@ -107,22 +108,6 @@ class TestCanonicalJson(unittest.TestCase):
             b'{\n    "la merde amus\xc3\xa9e": "\xF0\x9F\x92\xA9"\n}',
         )
 
-    @unittest.skipIf(
-        frozendict_type is None,
-        "If `frozendict` is not available, skip test",
-    )
-    def test_frozen_dict(self) -> None:
-        # For mypy's benefit:
-        assert frozendict_type is not None
-        self.assertEqual(
-            encode_canonical_json(frozendict_type({"a": 1})),
-            b'{"a":1}',
-        )
-        self.assertEqual(
-            encode_pretty_printed_json(frozendict_type({"a": 1})),
-            b'{\n    "a": 1\n}',
-        )
-
     def test_unknown_type(self) -> None:
         class Unknown(object):
             pass
@@ -167,3 +152,46 @@ class TestCanonicalJson(unittest.TestCase):
             from canonicaljson import json  # type: ignore[attr-defined]
 
             set_json_library(json)
+
+    def test_encode_unknown_class_raises(self) -> None:
+        class C:
+            pass
+
+        with self.assertRaises(Exception):
+            encode_canonical_json(C())
+
+    def test_preserialisation_callback(self) -> None:
+        class C:
+            pass
+
+        # Naughty: this alters the global state of the module. However this
+        # `C` class is limited to this test only, so this shouldn't affect
+        # other types and other tests.
+        register_preserialisation_callback(C, lambda c: "I am a C instance")
+
+        result = encode_canonical_json(C())
+        self.assertEqual(result, b'"I am a C instance"')
+
+    def test_cannot_register_preserialisation_callback_for_object(self) -> None:
+        with self.assertRaises(Exception):
+            register_preserialisation_callback(
+                object, lambda c: "shouldn't be able to do this"
+            )
+
+    def test_most_recent_preserialisation_callback_called(self) -> None:
+        class C:
+            pass
+
+        callback1 = Mock(return_value="callback 1 was called")
+        callback2 = Mock(return_value="callback 2 was called")
+
+        # Naughty: this alters the global state of the module. However this
+        # `C` class is limited to this test only, so this shouldn't affect
+        # other types and other tests.
+        register_preserialisation_callback(C, callback1)
+        register_preserialisation_callback(C, callback2)
+
+        encode_canonical_json(C())
+
+        callback1.assert_not_called()
+        callback2.assert_called_once()

Add support for immutabledict
https://github.com/matrix-org/synapse/pull/15113#issuecomment-1437447601
tests/test_canonicaljson.py::TestCanonicalJson::test_ascii tests/test_canonicaljson.py::TestCanonicalJson::test_cannot_register_preserialisation_callback_for_object tests/test_canonicaljson.py::TestCanonicalJson::test_encode_canonical tests/test_canonicaljson.py::TestCanonicalJson::test_encode_pretty_printed tests/test_canonicaljson.py::TestCanonicalJson::test_encode_unknown_class_raises tests/test_canonicaljson.py::TestCanonicalJson::test_invalid_float_values tests/test_canonicaljson.py::TestCanonicalJson::test_most_recent_preserialisation_callback_called tests/test_canonicaljson.py::TestCanonicalJson::test_preserialisation_callback tests/test_canonicaljson.py::TestCanonicalJson::test_set_json tests/test_canonicaljson.py::TestCanonicalJson::test_unknown_type