namespace error

"Namespace 'http://exslt.org/strings' does not contain any functions. "
	This is the message from the browser when loading an xsl file with this header declaration:
<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:str="http://exslt.org/strings"
                extension-element-prefixes="str">

				Thanks to everybody will pay attention to this my problem.