Cogserver.log reports Prompt has no substitution values....?

[cognosjon]

I've a prompt EngineType that has a parameter named pEngineType associated with it, the prompt is optional and multi select.

The report in which this prompt is contained functions correctly but within the Cogserver.log the following message is being continually displayed when the report is run.

Prompt'pEngineType' has no substitution values

anyone with any ideas?
Out of interest I get the same message if I select values within the prompt or leave the prompt blank to return all values.

thanks