diff --git a/synapse/config/cache.py b/synapse/config/cache.py
index bd9c5e5aaa..29338739b8 100644
--- a/synapse/config/cache.py
+++ b/synapse/config/cache.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 import os
-from typing import Dict
+from typing import Callable, Dict, Optional, Union
 
 from ._base import Config, ConfigError
 
@@ -22,7 +22,7 @@ _CACHES = {}
 _CACHE_PREFIX = "SYNAPSE_CACHE_FACTOR"
 
 # Wrap all global vars into a single object to eliminate `global` calls
-CACHE_PROPERTIES = {
+CACHE_PROPERTIES: Dict[str, Union[str, float, Optional[Callable[[], None]]]] = {
     "prefix": _CACHE_PREFIX,
     "default_size_factor": float(os.environ.get(_CACHE_PREFIX, 0.5)),
     # Callback to ensure that all caches are the correct size, registered when the
@@ -33,6 +33,7 @@ CACHE_PROPERTIES = {
 
 def add_resizable_cache(cache_name, cache_resize_callback):
     _CACHES[cache_name.lower()] = cache_resize_callback
+
     if CACHE_PROPERTIES["ensure_correct_cache_sizing"]:
         CACHE_PROPERTIES["ensure_correct_cache_sizing"]()