fill empty fo:cell with whitespace? (NEWBIE)

Hello
I have several
<fo:table-cell border-width="0.1mm" border-bottom-style="solid">
 <fo:block>
 </fo:block>
</fo:table-cell>

If the cell is empty, the underline is not an the same height as the 
cell before. Is there an way to fill it with like an &nbsp; ?
thanx
loebek


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