Ebcdic2ascii

I'm trying to modify an existing module and add some code to convert to ascii 
before writing to file.

This is the code:

==========

/* ebcdic-to-ascii */                                                         
void ebcdic2ascii(char *dest, const char *srce, size_t count);                
const unsigned char toascii[256] = {                                          
 0x00, 0x01, 0x02, 0x03, 0x85, 0x09, 0x86, 0x7f . . . .
. . . .
static tsize_t                                                   
_tiffWriteProc(thandle_t fd, tdata_t buf, tsize_t size)          
{                                                                
        tlong count = size;                                       
        ebcdic2ascii(buf, buf, count);                           
        return ((tsize_t) write((int) fd, buf, (size_t) size));  
}       
. . . . .
void                                                             
ebcdic2ascii(char *dest, const char *srce, size_t count)         
{                                                                
        while (count-- |= 0) {                                   
                *dest++ = toascii[*srce++];                      
        }                                                        
}                           

===============

when I compile I get the error: CZM0025  the operand must be a lvalue modifiable

What's wrong ?

Thanks for help.