RunInstances prepends the user name to the groupId field

If I issue a RunInstances request passing a security group name, like SecurityGroup.1=my_group, the response that I get prepends my user name to the security groupId:

  <groupSet>
    <item>
      <groupId>ekafree-my_group</groupId>
    </item>

The EC2 specification seems to require the plain security group name in that field, and Amazon EC2 indeed just puts the plain security group name, with no user name prepended. From the EC2 specification for RunInstancesResponse:

=======================
groupId
Name of the security group.

Type: xsd:string

Ancestor: item

Children: None
=======================

Thanks