diff --git a/src/core/core-varlink.c b/src/core/core-varlink.c
index b0b15d242c..71714e9473 100644
--- a/src/core/core-varlink.c
+++ b/src/core/core-varlink.c
@@ -234,7 +234,7 @@ static int vl_method_subscribe_managed_oom_cgroups(
         /* We only take one subscriber for this method so return an error if there's already an existing one.
          * This shouldn't happen since systemd-oomd is the only client of this method. */
         if (FLAGS_SET(flags, VARLINK_METHOD_MORE) && m->managed_oom_varlink)
-                return varlink_error(link, VARLINK_ERROR_SUBSCRIPTION_TAKEN, NULL);
+                return varlink_error(link, "io.systemd.ManagedOOM.SubscriptionTaken", NULL);
 
         r = build_managed_oom_cgroups_json(m, &v);
         if (r < 0)
diff --git a/src/shared/varlink-io.systemd.ManagedOOM.c b/src/shared/varlink-io.systemd.ManagedOOM.c
index 7cc5721dc2..d6414b3718 100644
--- a/src/shared/varlink-io.systemd.ManagedOOM.c
+++ b/src/shared/varlink-io.systemd.ManagedOOM.c
@@ -13,8 +13,11 @@ static VARLINK_DEFINE_METHOD(
                 SubscribeManagedOOMCGroups,
                 VARLINK_DEFINE_OUTPUT_BY_TYPE(cgroups, ControlGroup, VARLINK_ARRAY));
 
+static VARLINK_DEFINE_ERROR(SubscriptionTaken);
+
 VARLINK_DEFINE_INTERFACE(
                 io_systemd_ManagedOOM,
                 "io.systemd.ManagedOOM",
                 &vl_method_SubscribeManagedOOMCGroups,
-                &vl_type_ControlGroup);
+                &vl_type_ControlGroup,
+                &vl_error_SubscriptionTaken);
diff --git a/src/shared/varlink.h b/src/shared/varlink.h
index be4b328969..d49a8510f0 100644
--- a/src/shared/varlink.h
+++ b/src/shared/varlink.h
@@ -200,5 +200,4 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(VarlinkServer *, varlink_server_unref);
 #define VARLINK_ERROR_INVALID_PARAMETER "org.varlink.service.InvalidParameter"
 
 /* These are errors we came up with and squatted the namespace with */
-#define VARLINK_ERROR_SUBSCRIPTION_TAKEN "org.varlink.service.SubscriptionTaken"
 #define VARLINK_ERROR_PERMISSION_DENIED "org.varlink.service.PermissionDenied"