diff --git a/src/sparsezoo/requests/authentication.py b/src/sparsezoo/requests/authentication.py
index 8ed482ed..a96e85c0 100644
--- a/src/sparsezoo/requests/authentication.py
+++ b/src/sparsezoo/requests/authentication.py
@@ -35,7 +35,7 @@
 _LOGGER = logging.getLogger(__name__)
 
 NM_TOKEN_HEADER = "nm-token-header"
-AUTH_API = os.path.join(BASE_API_URL, "auth")
+AUTH_API = f"{BASE_API_URL}/auth"
 PUBLIC_AUTH_TYPE = "public"
 
 CREDENTIALS_YAML = os.path.abspath(