diff --git a/src/pdm/models/auth.py b/src/pdm/models/auth.py
index e43dcd4bee..890b8b974b 100644
--- a/src/pdm/models/auth.py
+++ b/src/pdm/models/auth.py
@@ -36,7 +36,7 @@ def _get_auth_from_index_url(self, netloc: str) -> tuple[MaybeAuth, str | None]:
             parsed = urllib.parse.urlparse(source["url"])
             auth, index_netloc = split_auth_from_netloc(parsed.netloc)
             if index_netloc == netloc:
-                if "username" in source:
+                if source.get("username"):
                     auth = (source["username"], source.get("password"))
                 return auth, source["url"]
         return None, None