[Zope] Simple problem with sql-group

[Zanotti Michele]

Hello,
I 'm a Zope newbye. I have a (probably silly) problem with this sql-template
(if you have already discussed this problem excuse me): 
        select * from employees 
        <dtml-sqlgroup where>
          <dtml-sqltest salary op=gt type=float optional>
        <dtml-and>
          <dtml-sqltest first op=eq type=string optional>
        <dtml-and>
          <dtml-sqltest last  op=eq type=string optional>
        </dtml-sqlgroup>  
My question is: 
if I pass the value 10000 to parameter salary (and nothing to others) I
expect to obtain the query:
	select * from employees
	where
	(salary > 10000)?
But I obtain the query
	select * from employees
	where
	(salary > 10000
	 and first = ''
	 and last = ''
	 ).
Is this result correct and am I wrong?
Can you help me? Thanks.
Michele