diff --git a/src/Encoder.php b/src/Encoder.php
index 3c206f46f..3d8b50257 100644
--- a/src/Encoder.php
+++ b/src/Encoder.php
@@ -427,7 +427,7 @@ public static function encodeClasses(array $classNames, $package = '')
     {
         $result = '';
         foreach ($classNames as $className) {
-            $result .= self::encodeClass($className, $package);
+            $result .= static::encodeClass($className, $package);
         }
 
         return $result;
diff --git a/src/Json.php b/src/Json.php
index ef76d817e..1c8bcd565 100644
--- a/src/Json.php
+++ b/src/Json.php
@@ -57,7 +57,7 @@ class Json
     public static function decode($encodedValue, $objectDecodeType = self::TYPE_OBJECT)
     {
         $encodedValue = (string) $encodedValue;
-        if (function_exists('json_decode') && self::$useBuiltinEncoderDecoder !== true) {
+        if (function_exists('json_decode') && static::$useBuiltinEncoderDecoder !== true) {
             $decode = json_decode($encodedValue, $objectDecodeType);
 
             switch (json_last_error()) {
@@ -105,7 +105,7 @@ public static function encode($valueToEncode, $cycleCheck = false, $options = ar
             if (method_exists($valueToEncode, 'toJson')) {
                 return $valueToEncode->toJson();
             } elseif (method_exists($valueToEncode, 'toArray')) {
-                return self::encode($valueToEncode->toArray(), $cycleCheck, $options);
+                return static::encode($valueToEncode->toArray(), $cycleCheck, $options);
             }
         }
 
@@ -114,11 +114,11 @@ public static function encode($valueToEncode, $cycleCheck = false, $options = ar
         if (isset($options['enableJsonExprFinder'])
            && ($options['enableJsonExprFinder'] == true)
         ) {
-            $valueToEncode = self::_recursiveJsonExprFinder($valueToEncode, $javascriptExpressions);
+            $valueToEncode = static::_recursiveJsonExprFinder($valueToEncode, $javascriptExpressions);
         }
 
         // Encoding
-        if (function_exists('json_encode') && self::$useBuiltinEncoderDecoder !== true) {
+        if (function_exists('json_encode') && static::$useBuiltinEncoderDecoder !== true) {
             $encodedResult = json_encode(
                 $valueToEncode,
                 JSON_HEX_TAG | JSON_HEX_APOS | JSON_HEX_QUOT | JSON_HEX_AMP
@@ -175,11 +175,11 @@ protected static function _recursiveJsonExprFinder(
             $value = $magicKey;
         } elseif (is_array($value)) {
             foreach ($value as $k => $v) {
-                $value[$k] = self::_recursiveJsonExprFinder($value[$k], $javascriptExpressions, $k);
+                $value[$k] = static::_recursiveJsonExprFinder($value[$k], $javascriptExpressions, $k);
             }
         } elseif (is_object($value)) {
             foreach ($value as $k => $v) {
-                $value->$k = self::_recursiveJsonExprFinder($value->$k, $javascriptExpressions, $k);
+                $value->$k = static::_recursiveJsonExprFinder($value->$k, $javascriptExpressions, $k);
             }
         }
         return $value;
@@ -229,23 +229,23 @@ protected static function _getXmlValue($simpleXmlElementObject)
     protected static function _processXml($simpleXmlElementObject, $ignoreXmlAttributes, $recursionDepth = 0)
     {
         // Keep an eye on how deeply we are involved in recursion.
-        if ($recursionDepth > self::$maxRecursionDepthAllowed) {
+        if ($recursionDepth > static::$maxRecursionDepthAllowed) {
             // XML tree is too deep. Exit now by throwing an exception.
             throw new RecursionException(
                 "Function _processXml exceeded the allowed recursion depth of "
-                .  self::$maxRecursionDepthAllowed
+                .  static::$maxRecursionDepthAllowed
             );
         }
 
         $children   = $simpleXmlElementObject->children();
         $name       = $simpleXmlElementObject->getName();
-        $value      = self::_getXmlValue($simpleXmlElementObject);
+        $value      = static::_getXmlValue($simpleXmlElementObject);
         $attributes = (array) $simpleXmlElementObject->attributes();
 
         if (!count($children)) {
             if (!empty($attributes) && !$ignoreXmlAttributes) {
                 foreach ($attributes['@attributes'] as $k => $v) {
-                    $attributes['@attributes'][$k] = self::_getXmlValue($v);
+                    $attributes['@attributes'][$k] = static::_getXmlValue($v);
                 }
                 if (!empty($value)) {
                     $attributes['@text'] = $value;
@@ -259,7 +259,7 @@ protected static function _processXml($simpleXmlElementObject, $ignoreXmlAttribu
         $childArray = array();
         foreach ($children as $child) {
             $childname = $child->getName();
-            $element   = self::_processXml($child, $ignoreXmlAttributes, $recursionDepth + 1);
+            $element   = static::_processXml($child, $ignoreXmlAttributes, $recursionDepth + 1);
             if (array_key_exists($childname, $childArray)) {
                 if (empty($subChild[$childname])) {
                     $childArray[$childname] = array($childArray[$childname]);
@@ -273,7 +273,7 @@ protected static function _processXml($simpleXmlElementObject, $ignoreXmlAttribu
 
         if (!empty($attributes) && !$ignoreXmlAttributes) {
             foreach ($attributes['@attributes'] as $k => $v) {
-                $attributes['@attributes'][$k] = self::_getXmlValue($v);
+                $attributes['@attributes'][$k] = static::_getXmlValue($v);
             }
             $childArray['@attributes'] = $attributes['@attributes'];
         }
@@ -323,11 +323,11 @@ public static function fromXml ($xmlStringContents, $ignoreXmlAttributes=true)
         $resultArray = null;
 
         // Call the recursive function to convert the XML into a PHP array.
-        $resultArray = self::_processXml($simpleXmlElementObject, $ignoreXmlAttributes);
+        $resultArray = static::_processXml($simpleXmlElementObject, $ignoreXmlAttributes);
 
         // Convert the PHP array to JSON using Zend_Json encode method.
         // It is just that simple.
-        $jsonStringOutput = self::encode($resultArray);
+        $jsonStringOutput = static::encode($resultArray);
         return($jsonStringOutput);
     }
 
diff --git a/src/Server/Response.php b/src/Server/Response.php
index 8365f626e..3c30b6be4 100644
--- a/src/Server/Response.php
+++ b/src/Server/Response.php
@@ -118,10 +118,10 @@ public function getResult()
     /**
      * Set result error
      *
-     * @param  Error $error
+     * @param  mixed $error
      * @return Response
      */
-    public function setError(Error $error)
+    public function setError(Error $error = null)
     {
         $this->error = $error;
         return $this;
diff --git a/src/Server/Smd.php b/src/Server/Smd.php
index 88a7801fe..669783b5e 100644
--- a/src/Server/Smd.php
+++ b/src/Server/Smd.php
@@ -382,7 +382,7 @@ public function toArray()
         $transport   = $this->getTransport();
         $envelope    = $this->getEnvelope();
         $contentType = $this->getContentType();
-        $SMDVersion  = self::SMD_VERSION;
+        $SMDVersion  = static::SMD_VERSION;
         $service = compact('transport', 'envelope', 'contentType', 'SMDVersion');
 
         if (null !== ($target = $this->getTarget())) {
diff --git a/test/Server/ResponseTest.php b/test/Server/ResponseTest.php
index 865d8619a..2cca7b5d2 100644
--- a/test/Server/ResponseTest.php
+++ b/test/Server/ResponseTest.php
@@ -67,6 +67,13 @@ public function testShouldBeAbleToRetrieveErrorObject()
         $this->assertSame($error, $this->response->getError());
     }
 
+    public function testErrorAccesorsShouldWorkWithNullInput()
+    {
+        $this->response->setError(null);
+        $this->assertNull($this->response->getError());
+        $this->assertFalse($this->response->isError());
+    }
+
     public function testIdShouldBeNullByDefault()
     {
         $this->assertNull($this->response->getId());