diff --git a/zerver/lib/alert_words.py b/zerver/lib/alert_words.py
index f9561e3d23..3466789681 100644
--- a/zerver/lib/alert_words.py
+++ b/zerver/lib/alert_words.py
@@ -11,7 +11,7 @@ from zerver.lib.cache import (
 from zerver.models import AlertWord, Realm, UserProfile, flush_realm_alert_words
 
 
-@cache_with_key(realm_alert_words_cache_key, timeout=3600 * 24)
+@cache_with_key(lambda realm: realm_alert_words_cache_key(realm.id), timeout=3600 * 24)
 def alert_words_in_realm(realm: Realm) -> Dict[int, List[str]]:
     user_ids_and_words = AlertWord.objects.filter(realm=realm, user_profile__is_active=True).values(
         "user_profile_id", "word"
@@ -23,7 +23,7 @@ def alert_words_in_realm(realm: Realm) -> Dict[int, List[str]]:
     return user_ids_with_words
 
 
-@cache_with_key(realm_alert_words_automaton_cache_key, timeout=3600 * 24)
+@cache_with_key(lambda realm: realm_alert_words_automaton_cache_key(realm.id), timeout=3600 * 24)
 def get_alert_word_automaton(realm: Realm) -> ahocorasick.Automaton:
     user_id_with_words = alert_words_in_realm(realm)
     alert_word_automaton = ahocorasick.Automaton()
@@ -66,7 +66,7 @@ def add_user_alert_words(user_profile: UserProfile, new_words: Iterable[str]) ->
         for word in word_dict.values()
     )
     # Django bulk_create operations don't flush caches, so we need to do this ourselves.
-    flush_realm_alert_words(user_profile.realm)
+    flush_realm_alert_words(user_profile.realm_id)
 
     return user_alert_words(user_profile)
 
diff --git a/zerver/lib/cache.py b/zerver/lib/cache.py
index d2916ad02d..1b44291706 100644
--- a/zerver/lib/cache.py
+++ b/zerver/lib/cache.py
@@ -635,8 +635,8 @@ def flush_realm(
         cache_delete(realm_user_dicts_cache_key(realm.id))
         cache_delete(active_user_ids_cache_key(realm.id))
         cache_delete(bot_dicts_in_realm_cache_key(realm.id))
-        cache_delete(realm_alert_words_cache_key(realm))
-        cache_delete(realm_alert_words_automaton_cache_key(realm))
+        cache_delete(realm_alert_words_cache_key(realm.id))
+        cache_delete(realm_alert_words_automaton_cache_key(realm.id))
         cache_delete(active_non_guest_user_ids_cache_key(realm.id))
         cache_delete(realm_rendered_description_cache_key(realm))
         cache_delete(realm_text_description_cache_key(realm))
@@ -645,12 +645,12 @@ def flush_realm(
         cache_delete(realm_text_description_cache_key(realm))
 
 
-def realm_alert_words_cache_key(realm: "Realm") -> str:
-    return f"realm_alert_words:{realm.string_id}"
+def realm_alert_words_cache_key(realm_id: int) -> str:
+    return f"realm_alert_words:{realm_id}"
 
 
-def realm_alert_words_automaton_cache_key(realm: "Realm") -> str:
-    return f"realm_alert_words_automaton:{realm.string_id}"
+def realm_alert_words_automaton_cache_key(realm_id: int) -> str:
+    return f"realm_alert_words_automaton:{realm_id}"
 
 
 def realm_rendered_description_cache_key(realm: "Realm") -> str:
diff --git a/zerver/models.py b/zerver/models.py
index bd37e4967a..64388397b3 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -4949,14 +4949,14 @@ class AlertWord(models.Model):
         unique_together = ("user_profile", "word")
 
 
-def flush_realm_alert_words(realm: Realm) -> None:
-    cache_delete(realm_alert_words_cache_key(realm))
-    cache_delete(realm_alert_words_automaton_cache_key(realm))
+def flush_realm_alert_words(realm_id: int) -> None:
+    cache_delete(realm_alert_words_cache_key(realm_id))
+    cache_delete(realm_alert_words_automaton_cache_key(realm_id))
 
 
 def flush_alert_word(*, instance: AlertWord, **kwargs: object) -> None:
-    realm = instance.realm
-    flush_realm_alert_words(realm)
+    realm_id = instance.realm_id
+    flush_realm_alert_words(realm_id)
 
 
 post_save.connect(flush_alert_word, sender=AlertWord)