diff --git a/nixos/modules/services/monitoring/parsedmarc.nix b/nixos/modules/services/monitoring/parsedmarc.nix
index 44fc359b6a7d..a146e7ab9543 100644
--- a/nixos/modules/services/monitoring/parsedmarc.nix
+++ b/nixos/modules/services/monitoring/parsedmarc.nix
@@ -301,6 +301,7 @@ in
               description = lib.mdDoc ''
                 The addresses to send outgoing mail to.
               '';
+              apply = x: if x == [] then null else lib.concatStringsSep "," x;
             };
           };