diff --git a/src/managers/BaseManager.js b/src/managers/BaseManager.js
index ae07ee62f..ea3717791 100644
--- a/src/managers/BaseManager.js
+++ b/src/managers/BaseManager.js
@@ -72,6 +72,10 @@ class BaseManager {
     if (typeof idOrInstance === 'string') return idOrInstance;
     return null;
   }
+
+  valueOf() {
+    return this.cache;
+  }
 }
 
 module.exports = BaseManager;
diff --git a/src/util/Util.js b/src/util/Util.js
index da4bff449..4ab518773 100644
--- a/src/util/Util.js
+++ b/src/util/Util.js
@@ -36,8 +36,10 @@ class Util {
       const elemIsObj = isObject(element);
       const valueOf = elemIsObj && typeof element.valueOf === 'function' ? element.valueOf() : null;
 
-      // If it's a collection, make the array of keys
+      // If it's a Collection, make the array of keys
       if (element instanceof require('./Collection')) out[newProp] = Array.from(element.keys());
+      // If the valueOf is a Collection, use its array of keys
+      else if (valueOf instanceof require('./Collection')) out[newProp] = Array.from(valueOf.keys());
       // If it's an array, flatten each element
       else if (Array.isArray(element)) out[newProp] = element.map(e => Util.flatten(e));
       // If it's an object with a primitive `valueOf`, use that value