Updating file permission to "owner" fails with 500 internal error - google-drive-api

Question

for our domain, there's a need to transfer files in a particular folder from one user to an admin account. The admin account has the "writer" role on all the files. Both users are in the same domain.
I'm using domain-wide delegation as the authorization method.
When updating the admin account's permission from "writer" to "owner", the program returns with 500 Internal Error. Any help will be appreciated. Thanks!
public static Drive getDriveService(String userEmail) throws GeneralSecurityException,IOException, URISyntaxException {
HttpTransport httpTransport = new NetHttpTransport();
JacksonFactory jsonFactory = new JacksonFactory();
Collection<String> scope = new ArrayList<String>();
scope.add(DriveScopes.DRIVE);
GoogleCredential credential = new GoogleCredential.Builder()
.setTransport(httpTransport)
.setJsonFactory(jsonFactory)
.setServiceAccountId(SERVICE_ACCOUNT_EMAIL)
.setServiceAccountScopes(scope)
.setServiceAccountUser(userEmail)
.setServiceAccountPrivateKeyFromP12File(
new java.io.File(SERVICE_ACCOUNT_PKCS12_FILE_PATH))
.build();
Drive service = new Drive.Builder(httpTransport, jsonFactory, null)
.setHttpRequestInitializer(credential).build();
return service;
}
public static void main(String[] args) {
try {
Drive userDriveService = getDriveService(ADMIN_ACCT_EMAIL);
ChildList list = userDriveService.children().list(folderId).setQ("not '" + ADMIN_ACCT_EMAIL + "' in owners").execute();
java.util.List<ChildReference> children = list.getItems();
//Simplified for testing
ChildReference child = children.get(0);
String fileId = child.getId();
String permId = userDriveService.permissions().getIdForEmail(ADMIN_ACCT_EMAIL).execute().getId();
Permission perm = userDriveService.permissions().get(fileId, permId).execute();
perm.setRole("owner");
userDriveService.permissions().update(fileId, permId, perm).setTransferOwnership(true).execute();
} catch (GeneralSecurityException | IOException | URISyntaxException e) {
e.printStackTrace();
}
}

Answer 1

Realized that I should have been using the owner's email to get the driveservice. This worked after changing that one line. Thanks.
Drive userDriveService = getDriveService(ORIGINAL_OWNER_EMAIL);