[xsl] Sort problem

["KrustyDerClown" <KrustyDerClown@xxxxxx>]

Hello,

i have a question about the xsl:sort function in XSLT.

I will sort a couple of data sets according to a number within this data 
sets.
I have the following code for this:

<xsl:sort select="pb_number" data-type="number"/>

It works fine, but my problem is that all data sets with an empty sequenze, 
instead of a number, stands on the beginning of the sorted list. But it 
should look like this:

Number
-----------
1
3
4
5
(data set with empty sequence)

Is that possible ? Any workaround for me ?

Thank you for your support.

Greets Oliver