diff --git a/tests/Monolog/Formatter/NormalizerFormatterTest.php b/tests/Monolog/Formatter/NormalizerFormatterTest.php
index 83c45ca6..095fb08e 100644
--- a/tests/Monolog/Formatter/NormalizerFormatterTest.php
+++ b/tests/Monolog/Formatter/NormalizerFormatterTest.php
@@ -182,7 +182,12 @@ class NormalizerFormatterTest extends \PHPUnit_Framework_TestCase
         $formatter = new NormalizerFormatter();
         $reflMethod = new \ReflectionMethod($formatter, 'toJson');
         $reflMethod->setAccessible(true);
-        $res = $reflMethod->invoke($formatter, array('message' => utf8_decode('öü')));
+
+        // send an invalid unicode sequence
+        $res = $reflMethod->invoke($formatter, array('message' => "\xB1\x31"));
+        if (PHP_VERSION_ID < 50500 && $res === array('message' => null)) {
+            throw new \RuntimeException('PHP 5.3/5.4 throw a warning and null the value instead of returning false entirely');
+        }
     }
 
     public function testExceptionTraceWithArgs()