background-image question

Interestingly enough, putting the background-image attribute as a "real"
attribute of fo:block in the transform produces the desired result.
However, adding it with an xsl:attribute element does not produce the
desired result.
I think I need to use xsl:attribute because I want to add/set the attribute
conditionally.

1.  works:

<fo:block background-image="draft.jpg">


2.  does not work:

<fo:block>
        <xsl:attribute name="background-image">
          <xsl:value-of select="draft.jpg"/>
        </xsl:attribute>

Any help is appreciated.
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