diff --git a/nixos/modules/system/boot/systemd-lib.nix b/nixos/modules/system/boot/systemd-lib.nix
index 9c8d4a026b4a..68a40377ee13 100644
--- a/nixos/modules/system/boot/systemd-lib.nix
+++ b/nixos/modules/system/boot/systemd-lib.nix
@@ -63,7 +63,7 @@ in rec {
 
   assertValueOneOf = name: values: group: attr:
     optional (attr ? ${name} && !elem attr.${name} values)
-      "Systemd ${group} field `${name}' cannot have value `${attr.${name}}'.";
+      "Systemd ${group} field `${name}' cannot have value `${toString attr.${name}}'.";
 
   assertHasField = name: group: attr:
     optional (!(attr ? ${name}))