"Invalid Class String" with new action

[SamS]

Hi

I've just written a user action but when I try running it I get an "Invalid Class String" message.

The exact message is :-
Error creating custom action 'UnbreakerSetRef': Invalid class string"

I can't see anything different to some user actions I wrote a couple of years ago. The content of the "Action" file was very slightly different to the old ones but I've made sure the format of both is the same, but this made no difference.

My new action is correctly registered (as far as I can see).

THis is VBP 6.3

Any suggestions on where to start looking?

Thanks

Sam