using exception instead of null return (preventing potential NPE)

DataMapper/UserMapper.php

@@ -70,7 +70,7 @@ class UserMapper
     {
         $result = $this->_adapter->find($id);
         if (0 == count($result)) {
-            return;
+            throw new \InvalidArgumentException("User #$id not found");
         }
         $row = $result->current();
 

Tests/DataMapper/UserMapperTest.php

@@ -87,4 +87,18 @@ class UserMapperTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals([$existing], $user);
     }
 
+    /**
+     * @expectedException InvalidArgumentException
+     * @expectedExceptionMessage User #404 not found
+     */
+    public function testNotFound()
+    {
+        $this->dbal->expects($this->once())
+                ->method('find')
+                ->with(404)
+                ->will($this->returnValue([]));
+
+        $user = $this->mapper->findById(404);
+    }
+
 }