[xsl] Type error with variable as=xs:string using xsl:choose

["Trevor Nicholls" <trevor@xxxxxxxxxxxxxxxxxx>]

The following variable definition is giving me the error
"XTTE0570: A sequence of more than one item is not allowed as the value of
variable normsrc"

I'm using Saxon 8.6.1 on Windows XP.

====
<xsl:variable name="normsrc" as="xs:string">
<xsl:choose>
<xsl:when test="contains($xsrc,'/images/images/')">
<xsl:text>images/</xsl:text>
<xsl:value-of select="substring-after($xsrc,'/images/images/')" />
</xsl:when>
<xsl:when test="contains($xsrc,'/images/')">
<xsl:text>images/</xsl:text>
<xsl:value-of select="substring-after($xsrc,'/images/')" />
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="$xsrc" />
</xsl:otherwise>
</xsl:choose>
</xsl:variable>
====

If I drop the as="xs:string" attribute it works perfectly, but I'm sure I've
got exactly this construction elsewhere without complaint. One difference
with this logic is that the 'when' tests overlap: any $xsrc that matches the
first 'when' will also match the second, but I understood that 'choose' will
only make one choice. Am I confused?

Cheers
Trevor