Incorrect string interpretation in xpath

[pgundlach]

<Layout xmlns="urn:speedata.de:2009/publisher/en"
  xmlns:sd="urn:speedata:2009/publisher/functions/en"
  version="3.1.24">
  
  <Record element="data">
    <PlaceObject>
      <Textblock>
        <Switch>
          <Case test="a = '*'">
            <Paragraph>
              <Value>x</Value>
            </Paragraph>
          </Case>
        </Switch>
      </Textblock>
    </PlaceObject>
  </Record>
</Layout>

This gives an error:

Error: The first operand of the multiplication is not a number. Evaluating to 0