The error trace is shown below.

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)

  quail-help("korean-hangul")

  eval((quail-help "korean-hangul") nil)

  eval-last-sexp-1(t)

  eval-last-sexp(t)

  eval-print-last-sexp()

  call-interactively(eval-print-last-sexp nil nil)

  recursive-edit()

  debug(error (wrong-type-argument char-or-string-p nil))

  quail-help("korean-hangul")

  eval((quail-help "korean-hangul") nil)

  eval-last-sexp-1(t)

  eval-last-sexp(t)

  eval-print-last-sexp()

  call-interactively(eval-print-last-sexp nil nil)

  recursive-edit()

  debug(error (void-function quail-help))

  (quail-help "korean-hangul")

  eval((quail-help "korean-hangul") nil)

  eval-last-sexp-1(t)

  eval-last-sexp(t)

  eval-print-last-sexp()

  call-interactively(eval-print-last-sexp nil nil)

  recursive-edit()

  debug(error (void-function quail-help))

  (quail-help (quote korean-hangul))

  eval((quail-help (quote korean-hangul)) nil)

  eval-last-sexp-1(t)

  eval-last-sexp(t)

  eval-print-last-sexp()

  call-interactively(eval-print-last-sexp nil nil)