diff --git a/include/ws_core.inc.php b/include/ws_core.inc.php
index cc3827394..3fa2bea39 100644
--- a/include/ws_core.inc.php
+++ b/include/ws_core.inc.php
@@ -586,7 +586,7 @@ Request format: ".@$this->_requestFormat." Response format: ".@$this->_responseF
     $result = trigger_change('ws_invoke_allowed', true, $methodName, $params);
     
     $is_error = false;
-    if (is_object($result) and strtolower( get_class($result) ) == 'pwgerror')
+    if ($result instanceof PwgError)
     {
       $is_error = true;
     }
diff --git a/include/ws_protocols/json_encoder.php b/include/ws_protocols/json_encoder.php
index b4b265d3b..45ac8827c 100644
--- a/include/ws_protocols/json_encoder.php
+++ b/include/ws_protocols/json_encoder.php
@@ -10,7 +10,7 @@ class PwgJsonEncoder extends PwgResponseEncoder
 {
   function encodeResponse($response)
   {
-    if (is_object($response) and strtolower(get_class($response)) == 'pwgerror')
+    if ($response instanceof PwgError)
     {
       return json_encode(
         array(
diff --git a/include/ws_protocols/php_encoder.php b/include/ws_protocols/php_encoder.php
index e2e5b9c91..9dd0cb508 100644
--- a/include/ws_protocols/php_encoder.php
+++ b/include/ws_protocols/php_encoder.php
@@ -10,8 +10,7 @@ class PwgSerialPhpEncoder extends PwgResponseEncoder
 {
   function encodeResponse($response)
   {
-    $respClass = strtolower( @get_class($response) );
-    if ($respClass=='pwgerror')
+    if ($response instanceof PwgError)
     {
       return serialize(
         array(
diff --git a/include/ws_protocols/xmlrpc_encoder.php b/include/ws_protocols/xmlrpc_encoder.php
index daf3e74cd..4ecb25d75 100644
--- a/include/ws_protocols/xmlrpc_encoder.php
+++ b/include/ws_protocols/xmlrpc_encoder.php
@@ -49,8 +49,7 @@ class PwgXmlRpcEncoder extends PwgResponseEncoder
 {
   function encodeResponse($response)
   {
-    $respClass = strtolower( @get_class($response) );
-    if ($respClass=='pwgerror')
+    if ($response instanceof PwgError)
     {
       $code = $response->code();
       $msg = htmlspecialchars($response->message());