XSLT error "not a node item"

I'm trying to create a nested hierarchy from a flat structure, and I am using the following key:

<xsl:key name="next-headings" 
              match="w:p[w:pPr/w:pStyle/@w:val = 'Heading3']" 
         use="generate-id((ancestor::w:sdt[1] | 
                           preceding-sibli开发者_运维百科ng::w:p
                                   [w:pPr/w:pStyle/@w:val = 'Heading1'] or 
                           preceding-sibling::w:p
                                   [w:pPr/w:pStyle/@w:val = 'Heading2']
                               )[last()])"/>

I am getting the "not a node item" error, but don't understand why. Any help in translating this error is much appreciated!

As @LarsH has answered, the or operator has more precedence that | union operator, and it results in a boolean expression that you can't union to a node set.

But, besides that it looks like you want to replace the or operator for a | union, I would use this expression:

generate-id((ancestor::w:sdt[1] |
             preceding-sibling::w:p
                [w:pPr/w:pStyle/@w:val[. = 'Heading1' or . = 'Heading2']]
            )[last()])

Edit: little typo...

I think you need to change or to |. You meant create a nodeset via union (|), but the or operator returns a boolean value, which is "not a node item".