diff --git a/src/Composer/Json/JsonManipulator.php b/src/Composer/Json/JsonManipulator.php
index 67020ffad..7099b7ffd 100644
--- a/src/Composer/Json/JsonManipulator.php
+++ b/src/Composer/Json/JsonManipulator.php
@@ -358,7 +358,7 @@ class JsonManipulator
 
     protected function detectIndenting()
     {
-        if ($this->pregMatch('{^(\s+)"}m', $this->contents, $match)) {
+        if ($this->pregMatch('{^([ \t]+)"}m', $this->contents, $match)) {
             $this->indent = $match[1];
         } else {
             $this->indent = '    ';
diff --git a/tests/Composer/Test/Json/JsonManipulatorTest.php b/tests/Composer/Test/Json/JsonManipulatorTest.php
index 26c82f28e..a2054e719 100644
--- a/tests/Composer/Test/Json/JsonManipulatorTest.php
+++ b/tests/Composer/Test/Json/JsonManipulatorTest.php
@@ -1112,6 +1112,28 @@ class JsonManipulatorTest extends \PHPUnit_Framework_TestCase
         "foo": "qux"
     }
 }
+', $manipulator->getContents());
+    }
+
+    public function testIndentDetection()
+    {
+        $manipulator = new JsonManipulator('{
+
+  "require": {
+    "php": "5.*"
+  }
+}');
+
+        $this->assertTrue($manipulator->addMainKey('require-dev', array('foo' => 'qux')));
+        $this->assertEquals('{
+
+  "require": {
+    "php": "5.*"
+  },
+  "require-dev": {
+    "foo": "qux"
+  }
+}
 ', $manipulator->getContents());
     }
 }