Amazon MWSAuthToken Error response

I am trying to retrieve all the MWSAuthToken as of the latest request from Amazon MWS https://images-na.ssl-images-amazon.com/images/G/01/mwsportal/doc/en_US/bde/MWSAuthToken.V325056062.pdf

I added a method to the Sellers API from https://developer.amazonservices.com/gp/mws/api.html/183-9742297-0917937?ie=UTF8&group=sellers&section=sellers&version=latest

And was able to recieve a valid response for other methods provided like ListMarketplaceParticipations and GetServiceStatus. But for each of my active SellerIds that I invoke the GetAuthToken I recieve Error response:

<?xml version="1.0"?>
<ErrorResponse xmlns="https://mws.amazonservices.com/Sellers/2011-07-01">
  <Error>
    <Type>Sender</Type>
    <Code>InvalidParameterValue</Code>
    <Message>Cannot request MWSAuthToken for own account</Message>
  </Error>
  <RequestId>b943a298-f49f-4562-b138-136b3b8f5e94</RequestId>
</ErrorResponse>

What can be the reason?

Page 5 of the first PDF (MWSAuthToken) that you reference states:

This operation is only intended for integrators or developers who have been authorized to make Amazon Marketplace Web Service (Amazon MWS) calls on a seller's behalf. If you are only using Amazon MWS to access your own seller account, or if you are using a desktop application, then you should not call this operation. If you do, you will receive a 400 error response.

I received the above Cannot request MWSAuthToken for own account error when I attempted GetAuthToken for our own account, but not when I did it for any of our customer Seller IDs for which we are authorized.