ant xmltask node if

my new problem is an if condition tags .

In fact, I can not get into the condition 'then' when "param" is 0.

<xmltask source="rapport.xml">                                      
    <call path="//testsuites">
        <param name="chaine" path="@name"/>
        <param name="testsuite1" path="testsuite[1]/@name"/>
        <param name="testsuite2" path="testsuite[2]/@name"/>                                
        <param name="ko1" path="count(testsuite[1]/testcase/failure[@type='ko'])"/>
        <actions>   
            <echo message="count fichiers ko 1 : @{ko1}"/>
            <if>
                <equals arg1="${ko1}" arg2="0" />
                <then>
                    <echo message="OK"/>
                </then>
                <else>                              
                    <echo message="KO"/>                            
                </else>
            </if>                           
        </actions>
    </call>                               
</xmltask>

Why I can not get into the "then" condition, but only in "else" while "arg1" = 0 ?