Effective boolean value is not defined for a sequence

hello,

just began to play with functions, but got error (see subject) at this
simple function


    <xsl:function name="local:path" as="node()">
        <xsl:param name="uri"/>
        <node><xsl:value-of select="string-join((tokenize($uri,'/')[1
to (last()-1)]), '/')"/></node>
    </xsl:function>

look like "last()-1" not acceptable here
any suggestions?

with best wishes,
Slav