RPG procedure *OMIT question

using v3r2m0,
i have a piece of code as shown below:

 *                                                          
 * procedure interface                                      
Dems9125r         pi                  opdesc                
D parlogdte                           like(loglogdte)  const
D parlogtim                           like(loglogtim)  const
D parmsgid                           like(logmsgid)   const
D partousr                             like(logtousr)   const
D                                            options(*omit:*nopass)
D partolan                            like(logtolan)   const
D                                           options(*omit:*nopass)
D parfrmusr                          like(logfrmusr)  const
D                                           options(*omit:*nopass)
D parfrmlan                          like(logfrmlan)  const
D                                           options(*omit:*nopass)

 *                                                                   
 * format 'send to' user                                             
 *                                                                   
0010 C                   if        %parms > 3                             
0020 C                   if        %addr(partousr) <> *null               
0030 C                   eval      logtousr  = partousr                   
0040 C                   endif                                            
0050 C                   endif   

the problem is that i get a compile-time error  "*RNF7421 30  Operands
are not compatible with the type of operator." pointing to line 0020
above (line numbers are my own, for illustration only). 

if i take out the CONST keyword for the parm 'partousr' in the procedure
interface shown above, the program compiles.

parm 'partousr' is a 10 long character field.  i would like the option of
NOT having to pass it, but if i do pass it i don't want it changed.  can
someone explain to me why this doesn't compile?

thank you.                                      




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