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diff --git a/subst.c b/subst.c
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--- a/subst.c
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+++ b/subst.c
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@@ -6240,7 +6240,7 @@ read_comsub (fd, quoted, flags, rflag)
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 	  /* read a multibyte character from buf */
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 	  /* punt on the hard case for now */
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 	  memset (&ps, '\0', sizeof (mbstate_t));
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-	  mblen = mbrtowc (&wc, bufp-1, bufn+1, &ps);
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+	  mblen = mbrtowc (&wc, bufp-1, bufn, &ps);
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 	  if (MB_INVALIDCH (mblen) || mblen == 0 || mblen == 1)
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 	    istring[istring_index++] = c;
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 	  else