diff --git a/src/Composer/Autoload/ClassMapGenerator.php b/src/Composer/Autoload/ClassMapGenerator.php
index d470ca2da..77dc08dc9 100644
--- a/src/Composer/Autoload/ClassMapGenerator.php
+++ b/src/Composer/Autoload/ClassMapGenerator.php
@@ -80,7 +80,7 @@ class ClassMapGenerator
 
             foreach ($classes as $class) {
                 if (array_key_exists($class, $map)) {
-                    throw new \RuntimeException('Ambiguous reference to class "'.$class.'" was found.');
+                    throw new \RuntimeException('Ambiguous class "'.$class.'" resolution; defined in "'.$map[$class].'" and in "'.$filePath.'" file.');
                 }
 
                 $map[$class] = $filePath;
diff --git a/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php b/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php
index fe33b1f36..58e68c0d5 100644
--- a/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php
+++ b/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php
@@ -106,7 +106,6 @@ class ClassMapGeneratorTest extends \PHPUnit_Framework_TestCase
 
     /**
      * @expectedException \RuntimeException
-     * @expectedExceptionMessage Ambiguous reference to class "A" was found.
      */
     public function testAmbiguousReference()
     {