diff --git a/subst.c b/subst.c

--- a/subst.c

+++ b/subst.c

@@ -6240,7 +6240,7 @@ read_comsub (fd, quoted, flags, rflag)

 	  /* read a multibyte character from buf */

 	  /* punt on the hard case for now */

 	  memset (&ps, '\0', sizeof (mbstate_t));

-	  mblen = mbrtowc (&wc, bufp-1, bufn+1, &ps);

+	  mblen = mbrtowc (&wc, bufp-1, bufn, &ps);

 	  if (MB_INVALIDCH (mblen) || mblen == 0 || mblen == 1)

 	    istring[istring_index++] = c;

 	  else