diff --git a/src/flareio/api_client.py b/src/flareio/api_client.py
index 161c049..de108e0 100644
--- a/src/flareio/api_client.py
+++ b/src/flareio/api_client.py
@@ -21,9 +21,11 @@
 
 
 _API_DOMAIN_DEFAULT: str = "api.flare.io"
-_ALLOWED_API_DOMAINS: t.Tuple[str, ...] = (
-    _API_DOMAIN_DEFAULT,
-    "api.eu.flare.io",
+_ALLOWED_API_DOMAINS: frozenset[str] = frozenset(
+    {
+        _API_DOMAIN_DEFAULT,
+        "api.eu.flare.io",
+    }
 )