<xsl:param name="param-id" select="/a/b/c[1]/@id"/>

I think you meant

  <xsl:param name="param-id" select=("/a/b/c)[1]/@id"/>

with the () you select 1 c node (thus 1 id attribute) without it
you select the first c node of each b child of a, so your param-id node
set contains severa nodes...

David

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