diff --git a/synapse/storage/client_ips.py b/synapse/storage/client_ips.py
index b01f0046e9..747d2df622 100644
--- a/synapse/storage/client_ips.py
+++ b/synapse/storage/client_ips.py
@@ -33,6 +33,7 @@ class ClientIpStore(background_updates.BackgroundUpdateStore):
         self.client_ip_last_seen = Cache(
             name="client_ip_last_seen",
             keylen=4,
+            max_entries=5000,
         )
 
         super(ClientIpStore, self).__init__(hs)