Cannot define a key in Schematron

Question

  <schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2"
            xmlns:xsl="https://www.w3.org/1999/XSL/Transform"
            xmlns="http://purl.oclc.org/dsdl/schematron">
    <title>FFM Marketplace Reconciliation</title>
    <xsl:key name="premiumPaidStatus" match="initialPremiumPaidStatus" use="../exchangeAssignedSubscriberId"/> 

If I use key from the xsl namespace like above I get

Warning: unrecognized element xsl:key

If I use key from the schematron namespace I get

Schema error: The key element is not in the ISO Schematron namespace. Use the XSLT namespace.