diff --git a/zerver/models.py b/zerver/models.py
index 5063201dde..77b9338820 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -108,8 +108,10 @@ def completely_open(domain):
 
 def get_unique_open_realm():
     # type: () -> Optional[Realm]
-    """We only return a realm if there is a unique non-system-only realm
-    and it is completely open."""
+    """We only return a realm if there is a unique non-system-only realm,
+    it is completely open, and there are no subdomains."""
+    if settings.REALMS_HAVE_SUBDOMAINS:
+        return None
     realms = Realm.objects.filter(deactivated=False)
     # On production installations, the (usually "zulip.com") system
     # realm is an empty realm just used for system bots, so don't