diff --git a/src/Symfony/Component/Serializer/Encoder/EncoderInterface.php b/src/Symfony/Component/Serializer/Encoder/EncoderInterface.php
index 8e5ebbc12f..d54da0bdd2 100644
--- a/src/Symfony/Component/Serializer/Encoder/EncoderInterface.php
+++ b/src/Symfony/Component/Serializer/Encoder/EncoderInterface.php
@@ -25,7 +25,7 @@ interface EncoderInterface
      * @param string $format  Format name
      * @param array  $context Options that normalizers/encoders have access to
      *
-     * @return string|int|float|bool
+     * @return string
      *
      * @throws UnexpectedValueException
      */
diff --git a/src/Symfony/Component/Serializer/Tests/Encoder/ChainEncoderTest.php b/src/Symfony/Component/Serializer/Tests/Encoder/ChainEncoderTest.php
index e7b02256e6..51c4f962dd 100644
--- a/src/Symfony/Component/Serializer/Tests/Encoder/ChainEncoderTest.php
+++ b/src/Symfony/Component/Serializer/Tests/Encoder/ChainEncoderTest.php
@@ -67,9 +67,9 @@ class ChainEncoderTest extends TestCase
     public function testEncode()
     {
         $this->encoder1->expects($this->never())->method('encode');
-        $this->encoder2->expects($this->once())->method('encode');
+        $this->encoder2->expects($this->once())->method('encode')->willReturn('foo:123');
 
-        $this->chainEncoder->encode(['foo' => 123], self::FORMAT_2);
+        $this->assertSame('foo:123', $this->chainEncoder->encode(['foo' => 123], self::FORMAT_2));
     }
 
     public function testEncodeUnsupportedFormat()