diff --git a/src/Monolog/Formatter/NormalizerFormatter.php b/src/Monolog/Formatter/NormalizerFormatter.php
index dfa3cb91..95b3de36 100644
--- a/src/Monolog/Formatter/NormalizerFormatter.php
+++ b/src/Monolog/Formatter/NormalizerFormatter.php
@@ -30,6 +30,9 @@ class NormalizerFormatter implements FormatterInterface
     public function __construct($dateFormat = null)
     {
         $this->dateFormat = $dateFormat ?: static::SIMPLE_DATE;
+        if (!function_exists('json_encode')) {
+            throw new \RuntimeException('PHP\'s json extension is required to use Monolog\'s NormalizerFormatter');
+        }
     }
 
     /**
diff --git a/src/Monolog/Handler/ChromePHPHandler.php b/src/Monolog/Handler/ChromePHPHandler.php
index ff7a7809..bc659349 100644
--- a/src/Monolog/Handler/ChromePHPHandler.php
+++ b/src/Monolog/Handler/ChromePHPHandler.php
@@ -50,6 +50,18 @@ class ChromePHPHandler extends AbstractProcessingHandler
 
     protected static $sendHeaders = true;
 
+    /**
+     * @param integer $level  The minimum logging level at which this handler will be triggered
+     * @param Boolean $bubble Whether the messages that are handled can bubble up the stack or not
+     */
+    public function __construct($level = Logger::DEBUG, $bubble = true)
+    {
+        parent::__construct($level, $bubble);
+        if (!function_exists('json_encode')) {
+            throw new \RuntimeException('PHP\'s json extension is required to use Monolog\'s ChromePHPHandler');
+        }
+    }
+
     /**
      * {@inheritdoc}
      */