diff --git a/src/JWK.php b/src/JWK.php
index 7632f4a4..29dbbac1 100644
--- a/src/JWK.php
+++ b/src/JWK.php
@@ -82,7 +82,7 @@ private static function parseKey(array $jwk)
 
         switch ($jwk['kty']) {
             case 'RSA':
-                if (\array_key_exists('d', $jwk)) {
+                if (!empty($jwk['d'])) {
                     throw new UnexpectedValueException('RSA private keys are not supported');
                 }
                 if (!isset($jwk['n']) || !isset($jwk['e'])) {
diff --git a/tests/JWKTest.php b/tests/JWKTest.php
index 93572400..0709836d 100644
--- a/tests/JWKTest.php
+++ b/tests/JWKTest.php
@@ -32,6 +32,36 @@ public function testInvalidAlgorithm()
         $keys = JWK::parseKeySet(array('keys' => array($badJwk)));
     }
 
+    public function testParsePrivateKey()
+    {
+        $this->setExpectedException(
+            'UnexpectedValueException',
+            'RSA private keys are not supported'
+        );
+        
+        $jwkSet = json_decode(
+            file_get_contents(__DIR__ . '/rsa-jwkset.json'),
+            true
+        );
+        $jwkSet['keys'][0]['d'] = 'privatekeyvalue';
+        
+        JWK::parseKeySet($jwkSet);
+    }
+    
+    public function testParseKeyWithEmptyDValue()
+    {
+        $jwkSet = json_decode(
+            file_get_contents(__DIR__ . '/rsa-jwkset.json'),
+            true
+        );
+        
+        // empty or null values are ok
+        $jwkSet['keys'][0]['d'] = null;
+        
+        $keys = JWK::parseKeySet($jwkSet);
+        $this->assertTrue(is_array($keys));
+    }
+
     public function testParseJwkKeySet()
     {
         $jwkSet = json_decode(