ACS20012 error when already signed into another account

Hi,

I'm using Azure ACS, using Office 365 as the identity provider - which is all working as it should with one exception: If I'm already logged into another account (such as the Azure portal on a separate live account), I get the following error:

HTTP Error Code: 400
Message: ACS20012: The request is not a valid WS-Federation protocol message.
Trace ID: c9e0294b-1ef2-4bfc-8f7b-0051b575a340
Timestamp: 2013-07-16 09:47:45Z

This URL: https://login.windows.net/33b7a355-01b7-47cf-9ff3-041f6b415684/wsfed?f=255&MSPPError=-2147205086

I would expect it to redirect to the error URL set in the 'Relying Party Application' config but it doesn't.

Am I missing something to get it to fail gracefully?

Thanks,

- Pete Stone.