'Clever' behaviour

If you query against a char index field with an integer, Informix (5.03) tries to be 'clever' and convert your char index to an integer index in a desparate attempt to satisfy the query. The biggest problem with this is the amount of time that it takes to do this. (Longer than a sequential scan would take!) My question is: Does anyone know how to turn off this feature? I would rather the server says 'not found' if an application accidently were to perform such a query. Mike.