[Saxon-JS 2] <xsl:call-template> param not passed within <ixsl:schedule-action>?

Hi,

I don't get how this can be happening, it's the same pattern which
I've used in a dozen other places -- supplying parameters to the
template called by <ixsl:schedule-action>.

I see the SELECT URI message being printed, so $select-uri cannot be
empty when it's supplied.

Yet this time Saxon-JS throws an error:

    code: "XTTE0600"
    message: "Required cardinality of value in 'xsl:param
name=\"Q{}select-uri\"' expression is exactly one; supplied value is
empty"
    name: "XError"
    oe: "869"

What could be the possible circumstances for this?

The code:

    <xsl:template name="apl:RootLoad">
        <xsl:context-item as="map(*)" use="required"/>

        <xsl:choose>
            <xsl:when test="?status = 200">
                <xsl:for-each select="?body">
                    <xsl:variable name="select-uri"
select="key('resources', $ldt:base)/dh:select/@rdf:resource"
as="xs:anyURI?"/>
                    <xsl:if test="$select-uri">
                        <xsl:message>SELECT URI: <xsl:value-of
select="$select-uri"/></xsl:message>
                        <ixsl:schedule-action http-request="map{
'method': 'GET', 'href': $select-uri, 'headers': map{ 'Accept':
'application/rdf+xml' } }">
                            <xsl:call-template
name="apl:RootChildrenSelectLoad">
                                <xsl:with-param name="select-uri"
select="$select-uri"/>
                            </xsl:call-template>
                        </ixsl:schedule-action>
                    </xsl:if>
                </xsl:for-each>
            </xsl:when>
            <xsl:otherwise>
            ...
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>

    <xsl:template name="apl:RootChildrenSelectLoad">
        <xsl:context-item as="map(*)" use="required"/>
        <xsl:param name="select-uri" as="xs:anyURI"/>

        ...


Martynas