Compile failed when try to do return not SUCCESSFUL where SUCCESSFUL is declared as const

D SUCCESSFUL      c                   const(*on)         
D proc1           pr              n                      
                                                         
C                   eval      *inlr = *on                
 *------------------------------------------------------ 
 * test return with not SUCCESSFUL                       
P proc1           B                                      
D                 pi              n                      
                                                         
C                   return    not SUCCESSFUL             
P proc1           E                                      

 
I'm getting this:
Msg id  Sv Number Seq     Message text

RNF7421 30     12 001200  Operands are not compatible with the type of
operator.

 
If I changed SUCCESSFUL to be a variable, it compile ok.   Any reason
why I can't do "return not constBoolean"?