diff --git a/classes/TheCure/Mappers/MockMapper.php b/classes/TheCure/Mappers/MockMapper.php
index 88de6d6..9183caa 100644
--- a/classes/TheCure/Mappers/MockMapper.php
+++ b/classes/TheCure/Mappers/MockMapper.php
@@ -122,7 +122,10 @@ function ($where) use ($collection)
 			{
 				if (isset($where['_id']))
 				{
-					return $collection[$where['_id']];
+					if (isset($collection[$where['_id']]))
+					{
+						return $collection[$where['_id']];
+					}
 				}
 				elseif ($where)
 				{
diff --git a/test/unit/classes/TheCure/Specs/Mappers/Mapper.php b/test/unit/classes/TheCure/Specs/Mappers/Mapper.php
index 8516849..ff600c9 100644
--- a/test/unit/classes/TheCure/Specs/Mappers/Mapper.php
+++ b/test/unit/classes/TheCure/Specs/Mappers/Mapper.php
@@ -89,6 +89,12 @@ public function testFindNone()
 		$this->assertNull($collection->current());
 	}
 
+	public function testFindOneWithNonExistentId()
+	{
+		$result = static::mapper()->findOne(123);
+		$this->assertNull($result);
+	}
+
 	public function provideModel()
 	{
 		$model = new Models\User;