Why is this CSV form letter code incompatable with the table of contents in ConTeXt?

Why is this CSV form letter code incompatable with the table of contents in ConTeXt?

Using the code in the answer to How to create form letters from spreadsheet data in ConTeXt?, I am building a document from data in a CSV file. I found that some part of the code is incomparable with chapter titles, such that, if I add a table of contents using \completecontent or add headers using \getmarkings{chapter}, it will not compile.
% macros=mkvi

\unprotect
\startluacode
  local datasets = { }

  local buffersraw   = buffers.raw
  local context      = context
  local ioloaddata   = io.loaddata
  local lpegmatch    = lpeg.match
  local stringformat = string.format
  local stringmatch  = string.match
  local stringsub    = string.sub
  local tableconcat  = table.concat
  local tableswapped = table.swapped

  local die = function (msg) print(msg or "ERROR") os.exit(1) end

  local csv_parser
  do
    --- This is (more than) an RFC 4180 parser.
    --- http://tools.ietf.org/html/rfc4180
    local C, Cg, Cs, Ct, P, S, V
        = lpeg.C, lpeg.Cg, lpeg.Cs, lpeg.Ct, lpeg.P, lpeg.S, lpeg.V

    local backslash = P[[\letterbackslash]]
    local comma     = ","
    local dquote    = P[["]]
    local eol       = S"\n\r"^1
    local noquote   = 1 - dquote
    local unescape  = function (s) return stringsub(s, 2) end
    csv_parser = P{
      "file",
      file    = Ct((V"header" * eol)^-1 * V"records"),
      header  = Cg(Ct(V"name" * (comma * V"name")^0), "header"),
      records = V"record" * (eol * V"record")^0 * eol^0,
      record  = Ct(V"field" * (comma * V"field")^0),
      name    = V"field",
      field   = V"escaped" + V"non_escaped",
      --- Deviate from rfc: the “textdata” terminal was defined only
      --- for 7bit ASCII. Also, any character may occur in a quoted
      --- field as long as it is escaped with a backslash. (\TEX\
      --- macros start with two backslashes.)
      escaped     = dquote
                  * Cs(((backslash * 1 / unescape) + noquote)^0)
                  * dquote
                  ,
      non_escaped = C((1 - dquote - eol - comma)^0),
    }
  end

  local process = function (id, raw)
    --- buffers may have trailing EOLs
    raw = stringmatch(raw, "^[\n\r]*(.-)[\n\r]*$")
    local data = lpegmatch(csv_parser, raw)
    --- map column name -> column nr
    data.header = tableswapped(data.header)
    datasets[id] = data
  end

  --- escaping hell ahead, please ignore.
  local s_item = [[
  \bgroup
    \string\def\string\insert{\string\getvalue{csv_insert_field}{%s}{%s}}%%
%s%% template
  \egroup
]]

  local typeset = function (id, template)
    local data   = datasets[id] or die("ERROR unknown dataset: " .. id)
    template     = stringmatch(buffersraw(template), "^[\n\r]*(.-)[\n\r]*$")
    local result = { }
    local last = \letterhash data
    for i=1, last do
      result[i] = stringformat(s_item, id, i, template)
    end
    context(tableconcat(result))
  end

  local insert = function (id, n, field)
    local this = datasets[id]
    context(this[n][this.header[field]])
  end

  commands.process_csv      = process
  commands.process_csv_file = function (id, fname)
    process(id, ioloaddata(fname, true))
  end
  commands.typeset_csv_job  = typeset
  commands