1 files changed, 2 insertions, 2 deletions
diff --git a/synapse/config/cache.py b/synapse/config/cache.py
index eb4194a5a9..015b2a138e 100644
--- a/synapse/config/cache.py
+++ b/synapse/config/cache.py
@@ -16,7 +16,7 @@ import logging
 import os
 import re
 import threading
-from typing import Any, Callable, Dict, Optional
+from typing import Any, Callable, Dict, Mapping, Optional
 
 import attr
 
@@ -94,7 +94,7 @@ def add_resizable_cache(
 
 class CacheConfig(Config):
     section = "caches"
-    _environ = os.environ
+    _environ: Mapping[str, str] = os.environ
 
     event_cache_size: int
     cache_factors: Dict[str, float]