Why does Test-ImapConnectivity fail with "The specified computer isn't a Client Access server." when I specify a machine that only has the client access role installed?

Hello

I have setup an Exchange 2013 CU6 environment with the following components:

• 1 machine with client access role   (exch2013ca)
• 3 machines with mailbox role
• 1 machine with both mailbox and client access role (exch2013cambx)

when I run Test-ImapConnectivity -ClientAccessServer exch2013ca in the Exchange Management Shell it fails with:

"...The specified computer isn't a Client Access server... " 

However when I run Test-ImapConnectivity -ClientAccessServer exch2013cambx  (where exch2013cambx is the machine that has both roles) it seems to work ok.