CactifyWarTask cannot be found

Hi,
I'm really confused with a problem with the cactifization ant task.
I've followed with previous related subjects here, but none of them
had my answer.

I'm using Cactus 1.7.2. there is a servlet sample inside the binary
dist. when I try to run the ant task, it gives me:

taskdef A class needed by class
org.apache.cactus.integration.ant.CactifyWarTask cannot be found:
org/codehaus/cargo/module/webapp/WarArchive

I'm sure that cargo.jar is in the class path since there is a path
defined in the build.xml:

  <path id="cactus.classpath">
    <path refid="project.classpath"/>
    <pathelement location="${aspectjrt.jar}"/>
    <pathelement location="${cactus.jar}"/>
    <pathelement location="${cactus.ant.jar}"/>
    <pathelement location="${cargo.jar}"/>
    <pathelement location="${commons.httpclient.jar}"/>
    <pathelement location="${commons.logging.jar}"/>
    <pathelement location="${junit.jar}"/>
  </path>

also ${carg.jar} points to a correct location.

here is the line which gives me error:
<taskdef resource="cactus.tasks" classpathref="cactus.classpath"/>

What is the problem?