diff --git a/lib/rocrate.js b/lib/rocrate.js
index 698ff39..260066f 100644
--- a/lib/rocrate.js
+++ b/lib/rocrate.js
@@ -216,7 +216,7 @@ class ROCrate {
     return true;
   }
 
-  __addValues(ref, prop, oldValues_, values, { duplicate = this.config.duplicate, replace = this.config.replace, merge = this.config.merge, recurse = false, seen }) {
+  __addValues(ref, prop, oldValues_, values, { duplicate = this.config.duplicate, replace = this.config.replace, merge = this.config.merge, recurse = false, seen = null }) {
     let oldValues = Utils.asArrayRef(oldValues_);
     const exists = (() => {
       if (duplicate) {