diff --git a/chai-immutable.js b/chai-immutable.js
index eee10e1..7a42985 100644
--- a/chai-immutable.js
+++ b/chai-immutable.js
@@ -24,6 +24,13 @@
 
     var Assertion = chai.Assertion;
 
+    function assertIsIterable(obj) {
+      new Assertion(obj).assert(
+        Immutable.Iterable.isIterable(obj),
+        'expected #{this} to be an Iterable'
+      );
+    }
+
     /**
      * ## BDD API Reference
      */
@@ -309,7 +316,7 @@
      */
 
     function assertCollectionSize(n) {
-      new Assertion(this._obj).instanceof(Collection);
+      assertIsIterable(this._obj);
 
       var size = this._obj.size;
       new Assertion(size).a('number');
@@ -335,7 +342,7 @@
     function assertCollectionSizeLeast(_super) {
       return function (n) {
         if (utils.flag(this, 'immutable.collection.size')) {
-          new Assertion(this._obj).instanceof(Collection);
+          assertIsIterable(this._obj);
 
           var size = this._obj.size;
           new Assertion(size).a('number');
@@ -355,7 +362,7 @@
     function assertCollectionSizeMost(_super) {
       return function (n) {
         if (utils.flag(this, 'immutable.collection.size')) {
-          new Assertion(this._obj).instanceof(Collection);
+          assertIsIterable(this._obj);
 
           var size = this._obj.size;
           new Assertion(size).a('number');
@@ -375,7 +382,7 @@
     function assertCollectionSizeAbove(_super) {
       return function (n) {
         if (utils.flag(this, 'immutable.collection.size')) {
-          new Assertion(this._obj).instanceof(Collection);
+          assertIsIterable(this._obj);
 
           var size = this._obj.size;
           new Assertion(size).a('number');
@@ -395,7 +402,7 @@
     function assertCollectionSizeBelow(_super) {
       return function (n) {
         if (utils.flag(this, 'immutable.collection.size')) {
-          new Assertion(this._obj).instanceof(Collection);
+          assertIsIterable(this._obj);
 
           var size = this._obj.size;
           new Assertion(size).a('number');
@@ -429,7 +436,7 @@
     Assertion.overwriteMethod('within', function (_super) {
       return function (min, max) {
         if (utils.flag(this, 'immutable.collection.size')) {
-          new Assertion(this._obj).instanceof(Collection);
+          assertIsIterable(this._obj);
 
           var size = this._obj.size;
           new Assertion(size).a('number');