It is strange you got this message because as you have noted one would have expected to get the error on a machine that was connected to a domain. Still it's worth trying the following group policy setting even if just to rule it out.
Go to "Start -> Run -> gpedit.msc" to open the Local Group Policy Editor. Now in the editor navigate to Local Computer Policy -> Computer Configuration -> Administrative Templates -> Windows Components -> Bitlocker Drive Encryption.
Open the policy setting called "Store Bitlocker recovery information in Active Directory Domain Services" and make sure that it is set to Not Configured.
If you read the description for that policy setting, it says that if you enable the policy and check the check-box for "Require Bitlocker backup to AD DS" then Bitlocker cannot be turned on unless the computer is 'connected to the domain and the backup of the Bitlocker recovery information to AD DS is successful'. So that could explain the cause of your issue.