Saxon

I get an error:

Error at char 18 in xsl:sequence/@select on line 32 column 60 of test.xslt:
  FORG0005: A sequence of more than one item is not allowed as the first argument of
  fn:exactly-one() (<a/>, <b/>) 
  at t:f() (...)
A sequence of more than one item is not allowed as the first argument of fn:exactly-one() (<a/>, <b/>) 

for a following stylesheet:

<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:map="http://www.w3.org/2005/xpath-functions/map"
  xmlns:t="t">
  
  <xsl:param name="items" as="element()*">
    <a pid="1"/>
    <b pid="1"/>
    <c pid="2"/>
    <d pid="2"/>
    <e pid="2"/>
  </xsl:param>
  
  <xsl:template match="/">
    <xsl:message select="t:f($items)"/>
  </xsl:template>
  
  <xsl:function name="t:f" as="xs:string*">
    <xsl:param name="items" as="element()*"/>

    <xsl:variable name="info" as="map(xs:string, element()+)" select="
      map:merge
      (
        $items!map { xs:string(@pid): . },
        map { 'duplicates': 'combine' }
      )"/>

    <xsl:variable name="keys" as="xs:string*" select="$info=>map:keys()"/>
      
    <xsl:sequence select="$keys!exactly-one($info(.)[1])"/>
  </xsl:function>

</xsl:stylesheet>

Input xml is anything.

In my opinion there should not be any error.