diff --git a/packages/ember-runtime/lib/mixins/array.js b/packages/ember-runtime/lib/mixins/array.js
index a2a01b37015..172afd719e4 100644
--- a/packages/ember-runtime/lib/mixins/array.js
+++ b/packages/ember-runtime/lib/mixins/array.js
@@ -205,7 +205,7 @@ export default Mixin.create(Enumerable, {
 
   firstObject: computed(function() {
     return objectAt(this, 0);
-  }),
+  }).readOnly(),
 
   lastObject: computed(function() {
     return objectAt(this, get(this, 'length') - 1);
diff --git a/packages/ember-runtime/lib/mixins/enumerable.js b/packages/ember-runtime/lib/mixins/enumerable.js
index e8b9a8b9ee5..6be73a26f82 100644
--- a/packages/ember-runtime/lib/mixins/enumerable.js
+++ b/packages/ember-runtime/lib/mixins/enumerable.js
@@ -153,6 +153,7 @@ export default Mixin.create({
 
     @property firstObject
     @return {Object} the object or undefined
+    @readOnly
     @public
   */
   firstObject: computed('[]', function() {
@@ -167,7 +168,7 @@ export default Mixin.create({
     pushCtx(context);
 
     return ret;
-  }),
+  }).readOnly(),
 
   /**
     Helper method returns the last object from a collection. If your enumerable
diff --git a/packages/ember-runtime/tests/suites/enumerable/firstObject.js b/packages/ember-runtime/tests/suites/enumerable/firstObject.js
index 28372de5c7b..822d4be1aed 100644
--- a/packages/ember-runtime/tests/suites/enumerable/firstObject.js
+++ b/packages/ember-runtime/tests/suites/enumerable/firstObject.js
@@ -1,5 +1,6 @@
 import {SuiteModuleBuilder} from 'ember-runtime/tests/suites/suite';
 import {get} from 'ember-metal/property_get';
+import {set} from 'ember-metal/property_set';
 
 var suite = SuiteModuleBuilder.create();
 
@@ -15,4 +16,14 @@ suite.test('returns undefined if enumerable is empty', function() {
   equal(get(obj, 'firstObject'), undefined);
 });
 
+suite.test('can not be set', function() {
+  var obj = this.newObject([]);
+
+  equal(get(obj, 'firstObject'), this.toArray(obj)[0]);
+
+  throws(function() {
+    set(obj, 'firstObject', 'foo!');
+  }, /Cannot set read-only property "firstObject" on object/);
+});
+
 export default suite;