relativePath not working

[Davis Ford]

Hi, I'm using mvn 2.0.6

I have a simple structure

parent_project
   ----sub_project
      ----pom.xml
   ----pom.xml

parent_project has this:

   <groupId>my.org</groupId>
   <artifactId>parent</artifactId>
   <version>1.0-SNAPSHOT</version>
   <packaging>pom</packaging>

sub_project has this:

   <parent>
       <groupId>my.org</groupId>
       <artifactId>parent</artifactId>
       <version>1.0-SNAPSHOT</version>
       <relativePath>../pom.xml</relativePath>
   </parent>
   <groupId>my.org</groupId>
   <artifactId>sub</artifactId>
   <packaging>pom</packaging>
   <version>1.0-SNAPSHOT</version>

anytime i issue any mvn command in the sub_project dir, i get:
org.apache.maven.reactor.MavenExecutionException: Cannot find parent: etc...

how can i get relativePath to work?

thx,
davis