The SAS KCOMPRESS, COMPRESS, and SCAN functions support null arguments. However, SAS Rules Studio reports a syntax error when an optional argument is not set to a value. 

Here is an example that calls the COMPRESS function with a null second argument: 

%set( _A_COMPRESS_KAD ) = COMPRESS( 'test     123', , 'kad' ); 

The error reported by the syntax checker is as follows:

line x:x  text is invalid or used out of proper order: ‘,’ 

There is no workaround for this issue.