diff --git a/changelog.d/11832.misc b/changelog.d/11832.misc
new file mode 100644
index 0000000000000000000000000000000000000000..5ff117d93326f05262924c73b869cc31b8374506
--- /dev/null
+++ b/changelog.d/11832.misc
@@ -0,0 +1 @@
+Fix type errors introduced by new annotations in the Prometheus Client library.
\ No newline at end of file
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 9e6c1b2f3b54e74b1cb6126750b51c0533df075b..cca084c18c2112a658a8f0ffe8b0db25dfe1f374 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -30,6 +30,7 @@ from typing import (
     Type,
     TypeVar,
     Union,
+    cast,
 )
 
 import attr
@@ -60,7 +61,7 @@ all_gauges: "Dict[str, Union[LaterGauge, InFlightGauge]]" = {}
 HAVE_PROC_SELF_STAT = os.path.exists("/proc/self/stat")
 
 
-class RegistryProxy:
+class _RegistryProxy:
     @staticmethod
     def collect() -> Iterable[Metric]:
         for metric in REGISTRY.collect():
@@ -68,6 +69,13 @@ class RegistryProxy:
                 yield metric
 
 
+# A little bit nasty, but collect() above is static so a Protocol doesn't work.
+# _RegistryProxy matches the signature of a CollectorRegistry instance enough
+# for it to be usable in the contexts in which we use it.
+# TODO Do something nicer about this.
+RegistryProxy = cast(CollectorRegistry, _RegistryProxy)
+
+
 @attr.s(slots=True, hash=True, auto_attribs=True)
 class LaterGauge:
 
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index 80786464c2fbc49bb475417d11ef2b3988b5771f..22b4606ae0ea2397da1b3f53967b0558e732b3d4 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -76,8 +76,7 @@ REQUIREMENTS = [
     "msgpack>=0.5.2",
     "phonenumbers>=8.2.0",
     # we use GaugeHistogramMetric, which was added in prom-client 0.4.0.
-    # 0.13.0 has an incorrect type annotation, see #11832.
-    "prometheus_client>=0.4.0,<0.13.0",
+    "prometheus_client>=0.4.0",
     # we use `order`, which arrived in attrs 19.2.0.
     # Note: 21.1.0 broke `/sync`, see #9936
     "attrs>=19.2.0,!=21.1.0",