CDF doneProc callback no longer runs.

I have a symbol that sets some CDF parameters using a callback with the doneProc event.

I now get the following error "*WARNING* An error occurred while applying the CDF doneProc 'X' to (db:0x17f5a99a)"

(X is put in place of the name of the callback)

The error is quite vague so I'm not sure what to do