isolated high surrogate

I'm stumped by this error:
org.xml.sax.SAXParseException; lineNumber: 92; columnNumber: 51; java.lang.IllegalArgumentException: isolated high surrogate

I have text with surrogate pairs throughout the file, but this only occurs in this context:
    <fo:block padding-top="2em" padding-bottom=".5em" text-align="left" font-family="Kozuka Gothic PR6N" font-size="18pt" color="black">
      <xsl:call-template name="zero_width_space_1">
        <xsl:with-param name="data" select="@documentName"/>
      </xsl:call-template>
    </fo:block>

I've checked the input stream, and all the surrogates are correctly paired. I've tried escaping the surrogate pairs (e.g. "&#-integer-;"), but that doesn't change the error.