Crystal Reports Make a Number Parameter optional when running repo

Message

   I have a report with a number parameter and want to be able to provide the user the option to not select that parameter when running the report.
Report for instance has 4 parameters, Work Order #, Activity, Start and End Date
The Work Order number is a Number type parameter

I tried accomplishing this my doing the following:  1)Changed the date type of the WO# parameter from number to String 2)  changing the code in the Report Selection formula by adding ToText around the field that is a number.

IF {@IF {@ 
IF NOT{@WONOisBlank}and {@ActivityIsBlank} and 
   {@StartDateIsBlank} and {@EndDateIsBlank} THEN
   ToText({HISTORY.HISTKEY})  = {?WONO}

The report does not return any data an I am not sure what is wrong.  Any clues would be appreciated.  TY in advance.

IF {@ 