/tmp $ cat tmp.scm 
(import (scheme base) (scheme write))

;;; Continuation byte.
(write (map char->integer (string->list
                           (utf8->string
                            #u8(#x80)))))
(newline)

/tmp $ valgrind -s --track-origins=yes ~/.local/chicken-master/bin/csi -n -s tmp.scm 
==9420== Memcheck, a memory error detector
==9420== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==9420== Using Valgrind-3.25.1 and LibVEX; rerun with -h for copyright info
==9420== Command: /home/user/.local/chicken-master/bin/csi -n -s tmp.scm
==9420== 
==9420== Conditional jump or move depends on uninitialised value(s)
==9420==    at 0x5145D77: utf8_decode (utf.c:3206)
==9420==    by 0x514909B: C_utf_validate (utf.c:3525)
==9420==    by 0x4A721F8: f_22866 (library.c:34762)
==9420==    by 0x4A72058: f_22841 (library.c:34747)
==9420==    by 0x4C06C19: f_7588 (eval.c:18418)
==9420==    by 0x4BC9387: f_4236 (eval.c:10185)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==  Uninitialised value was created by a stack allocation
==9420==    at 0x5131ED7: allocate_bytevector_2 (runtime.c:10307)
==9420== 
==9420== Conditional jump or move depends on uninitialised value(s)
==9420==    at 0x5145D9D: utf8_decode (utf.c:3207)
==9420==    by 0x514909B: C_utf_validate (utf.c:3525)
==9420==    by 0x4A721F8: f_22866 (library.c:34762)
==9420==    by 0x4A72058: f_22841 (library.c:34747)
==9420==    by 0x4C06C19: f_7588 (eval.c:18418)
==9420==    by 0x4BC9387: f_4236 (eval.c:10185)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==  Uninitialised value was created by a stack allocation
==9420==    at 0x5131ED7: allocate_bytevector_2 (runtime.c:10307)
==9420== 
==9420== Conditional jump or move depends on uninitialised value(s)
==9420==    at 0x5145DC4: utf8_decode (utf.c:3208)
==9420==    by 0x514909B: C_utf_validate (utf.c:3525)
==9420==    by 0x4A721F8: f_22866 (library.c:34762)
==9420==    by 0x4A72058: f_22841 (library.c:34747)
==9420==    by 0x4C06C19: f_7588 (eval.c:18418)
==9420==    by 0x4BC9387: f_4236 (eval.c:10185)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==  Uninitialised value was created by a stack allocation
==9420==    at 0x5131ED7: allocate_bytevector_2 (runtime.c:10307)
==9420== 

Error: (utf8->string) string contains invalid UTF-8 sequence: #u8(128)

	Call history:

	<syntax>	 (##core#quote #f)
	tmp.scm:1	 (##core#require r7lib scheme.write)
	<syntax>	 (chicken.load#load-unit (##core#quote r7lib) (##core#quote #f) (##core#quote #f))
	<syntax>	 (##core#quote r7lib)
	<syntax>	 (##core#quote #f)
	<syntax>	 (##core#quote #f)
	<eval>	 (chicken.load#load-unit (##core#quote library) (##core#quote #f) (##core#quote #f))
	<eval>	 (chicken.load#load-unit (##core#quote r7lib) (##core#quote #f) (##core#quote #f))
	tmp.scm:4	 (write (map char->integer (string->list (utf8->string #u8(128)))))
	tmp.scm:4	 (map char->integer (string->list (utf8->string #u8(128))))
	tmp.scm:4	 (string->list (utf8->string #u8(128)))
	tmp.scm:5	 (utf8->string #u8(128))
	tmp.scm:4	 (write (map char->integer (string->list (utf8->string #u8(128)))))
	tmp.scm:4	 (map char->integer (string->list (utf8->string #u8(128))))
	tmp.scm:4	 (string->list (utf8->string #u8(128)))
	tmp.scm:5	 (utf8->string #u8(128))	<--
==9420== 
==9420== HEAP SUMMARY:
==9420==     in use at exit: 3,358,039 bytes in 4,110 blocks
==9420==   total heap usage: 4,113 allocs, 3 frees, 4,406,639 bytes allocated
==9420== 
==9420== LEAK SUMMARY:
==9420==    definitely lost: 0 bytes in 0 blocks
==9420==    indirectly lost: 0 bytes in 0 blocks
==9420==      possibly lost: 0 bytes in 0 blocks
==9420==    still reachable: 3,358,039 bytes in 4,110 blocks
==9420==         suppressed: 0 bytes in 0 blocks
==9420== Rerun with --leak-check=full to see details of leaked memory
==9420== 
==9420== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)
==9420== 
==9420== 1 errors in context 1 of 3:
==9420== Conditional jump or move depends on uninitialised value(s)
==9420==    at 0x5145DC4: utf8_decode (utf.c:3208)
==9420==    by 0x514909B: C_utf_validate (utf.c:3525)
==9420==    by 0x4A721F8: f_22866 (library.c:34762)
==9420==    by 0x4A72058: f_22841 (library.c:34747)
==9420==    by 0x4C06C19: f_7588 (eval.c:18418)
==9420==    by 0x4BC9387: f_4236 (eval.c:10185)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==  Uninitialised value was created by a stack allocation
==9420==    at 0x5131ED7: allocate_bytevector_2 (runtime.c:10307)
==9420== 
==9420== 
==9420== 1 errors in context 2 of 3:
==9420== Conditional jump or move depends on uninitialised value(s)
==9420==    at 0x5145D9D: utf8_decode (utf.c:3207)
==9420==    by 0x514909B: C_utf_validate (utf.c:3525)
==9420==    by 0x4A721F8: f_22866 (library.c:34762)
==9420==    by 0x4A72058: f_22841 (library.c:34747)
==9420==    by 0x4C06C19: f_7588 (eval.c:18418)
==9420==    by 0x4BC9387: f_4236 (eval.c:10185)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==  Uninitialised value was created by a stack allocation
==9420==    at 0x5131ED7: allocate_bytevector_2 (runtime.c:10307)
==9420== 
==9420== 
==9420== 1 errors in context 3 of 3:
==9420== Conditional jump or move depends on uninitialised value(s)
==9420==    at 0x5145D77: utf8_decode (utf.c:3206)
==9420==    by 0x514909B: C_utf_validate (utf.c:3525)
==9420==    by 0x4A721F8: f_22866 (library.c:34762)
==9420==    by 0x4A72058: f_22841 (library.c:34747)
==9420==    by 0x4C06C19: f_7588 (eval.c:18418)
==9420==    by 0x4BC9387: f_4236 (eval.c:10185)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==    by 0x4C06AA8: f_7581 (eval.c:18395)
==9420==    by 0x4BC66AB: f_4001 (eval.c:9811)
==9420==    by 0x4C068A4: f_7574 (eval.c:18370)
==9420==  Uninitialised value was created by a stack allocation
==9420==    at 0x5131ED7: allocate_bytevector_2 (runtime.c:10307)
==9420== 
==9420== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)