diff --git a/chai-immutable.js b/chai-immutable.js
index 1adb128..da7e86f 100644
--- a/chai-immutable.js
+++ b/chai-immutable.js
@@ -24,6 +24,10 @@
 
     var Assertion = chai.Assertion;
 
+    function isImmutable(obj) {
+      return Immutable.Iterable.isIterable(obj);
+    }
+
     /**
      * ## BDD API Reference
      */
@@ -46,7 +50,7 @@
       return function () {
         var obj = this._obj;
 
-        if (obj && obj instanceof Collection) {
+        if (isImmutable(obj)) {
           var size = obj.size;
           new Assertion(size).a('number');
 
@@ -96,7 +100,7 @@
       return function (collection) {
         var obj = this._obj;
 
-        if (obj && obj instanceof Collection) {
+        if (isImmutable(obj)) {
           this.assert(
             Immutable.is(obj, collection),
             'expected #{act} to equal #{exp}',
@@ -141,7 +145,7 @@
       return function (val) {
         var obj = this._obj;
 
-        if (obj && obj instanceof Collection) {
+        if (isImmutable(obj)) {
           this.assert(
             obj.includes(val),
             'expected #{act} to include #{exp}',