Saxon error: "If none of the conditions is satisfied,

Hello, 

When attempting to compile a stylesheet containing the following
variable 
definition:

<xsl:variablename="seq_position"as="xs:integer*">
 <xsl:choose>
   <xsl:whentest="name() eq 'step1'">
      
<xsl:sequenceselect="count(preceding-sibling::step1) + 1"/>
   </xsl:when>
  
<xsl:whentest="name() eq 'step2'">
      
<xsl:sequenceselect="count(preceding-sibling::step2) + 1"/>
   </xsl:when>
  
<xsl:whentest="name() eq 'step3'">
      
<xsl:sequenceselect="count(preceding-sibling::step3) + 1"/>
   </xsl:when>
  
<xsl:otherwise>
       <xsl:sequenceselect="99+1"/><!--trying to placate
saxon->
   </xsl:otherwise>
 </xsl:choose> 
</xsl:variable>
 
I get the error:
"XTTE0570: Conditional expession: If none of the conditions is satisfied, an
empty sequence
  will be returned, but this is not allowed as the value of
variable 
$seq_position"
 
I haven't the foggiest idea why this would be. Any
help would be most greatly 
appreciated.