diff --git a/nixos/modules/services/monitoring/prometheus/default.nix b/nixos/modules/services/monitoring/prometheus/default.nix
index a07445ce167c..cf9deccbffe2 100644
--- a/nixos/modules/services/monitoring/prometheus/default.nix
+++ b/nixos/modules/services/monitoring/prometheus/default.nix
@@ -134,6 +134,7 @@ let
           };
         });
         default = null;
+        apply = x: if x == null then null else _filter x;
         description = ''
           Optional http login credentials for metrics scraping.
         '';