I think you are using 'httpResult.Responseheader.set-cookie' instead of 'httpResult.Responseheader['set-cookie'].'
The mentioned error would be thrown on any version of ColdFusion when you have a hyphen in the key. To address this the key must be quoted inside square brackets i.e. structVariable['key'].
Can you share the code that works on CF9?
Adobe ColdFusion Team.