If statement when块中的xslt-if条件

为什么dos条件在when块中不起作用

    <xsl:choose>
      <xsl:when test="type = 'news'">
        <xsl:if test="string-length(subtitle) > 0">
          <filter_subtitle><i>In <xsl:value-of select="subtitle"/></i></filter_subtitle>
        </xsl:if>
      </xsl:when>
      <xsl:otherwise>
        <filter_subtitle><xsl:value-of select="subtitle"/></filter_subtitle>
      </xsl:otherwise>
    </xsl:choose>

很好

<xsl:when test="type='news' and string-length(subtitle)>0">