diff --git a/lib/util/Collections.js b/lib/util/Collections.js
index <HASH>..<HASH> 100644
--- a/lib/util/Collections.js
+++ b/lib/util/Collections.js
@@ -6,22 +6,21 @@
  * @param  {Array<Object>} [collection]
  * @param  {Object} [element]
  *
- * @return {Object} the element that got removed or undefined
+ * @return {Number} the previous index of the element
  */
 module.exports.remove = function(collection, element) {
 
   if (!collection || !element) {
-    return;
+    return -1;
   }
 
   var idx = collection.indexOf(element);
-  if (idx === -1) {
-    return;
-  }
 
-  collection.splice(idx, 1);
+  if (idx !== -1) {
+    collection.splice(idx, 1);
+  }
 
-  return element;
+  return idx;
 };
 
 /**
@@ -38,7 +37,7 @@ module.exports.add = function(collection, element, idx) {
     return;
   }
 
-  if (isNaN(idx)) {
+  if (typeof idx !== 'number') {
     idx = -1;
   }
 
@@ -72,7 +71,7 @@ module.exports.add = function(collection, element, idx) {
 
 
 /**
- * Fail get the index of an element in a collection.
+ * Fail save get the index of an element in a collection.
  *
  * @param {Array<Object>} collection
  * @param {Object} element
feat(util/Collections): return old index #remove