XPathSelectElement with namespace return error - want to read element at any depth

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XML:

<response xmlns="abc">
	<errors>
		<error>Authentication failed</error>
	</errors>
</response>

Code:

XNamespace ns = "abc";
XmlNamespaceManager namespaceManager = new XmlNamespaceManager(new NameTable());
namespaceManager.AddNamespace(string.Empty, "abc");
string err = XDOC.Element(ns + "response").XPathSelectElement("errors/error", namespaceManager).Value;

but it gives me error "object reference not set to instance of an object"

please help.

it working fine if i used,
string err = XDOC.Element(ns + "response").Element(ns + "errors").Element(ns+"error").Value;

but i want to use XPathSelectElement.

thanks in advance..