more than one "Permission Scheme" being set when use soap

i use getProjects get my project(why not have a getProject method?), then

nullPerm = new RemotePermissionScheme();
nullPerm.setId(new Long(nullPermID));

myProject.setPermissionScheme(nullPerm);

jiraSoapService.updateProject(token, myProject);

but got two permissionScheme for my project,very strange!