error: data-type on xsl:sort must be 'text' or 'number

Saxon 8b, xslt 2, Oxygen 8.1

I thought data-type and order attributes were attribute value templates?
I don't know if this error is generated by Saxon or by Oxygen, but if
the error is valid, what do I have wrong?

<xsl:sort select="if ($keys/k[1]/@val) then
sax:evaluate($keys/k[1]/@val) else 'foo'" data-type="{$keys/k[1]/@type}"
order="{$keys/k[1]/@order}"/>

Thanks!
Angela Williams
Software Developer
The 401k Company, A Charles Schwab Company
98 San Jacinto Blvd. ~ Suite 1100 ~ Austin, TX 78701
Office: 512.344.1547 ~ Fax: 512.397.6656
Angela.Williams@xxxxxxxxxxxxxxxxxx