I have a custom block showing the users whose content has attracted most pageviews.

part of my code is this:

print "<tr class=\"";
print $zebra;
print "\"><td>";

this is a drupal 6 installation and I don't seem to be able to get the odd/even classes I'm looking for. If I put print "odd", they all come out grey, "even" and they all come out white, so I know the syntax of the code is fine...I just can't get the zebra variable working.

any tips what I'm failing to do??