struct field not writable

I wanted to add my own file descriptor to the glib main loop.
Trying to create a "SourceFuncs":

local GLib = require"lgi".GLib
GLib.SourceFuncs {
    prepare = print;
    check = print;
    dispatch = print;
    finalize = print
}

/usr/share/lua/5.3/lgi/record.lua:94: GLib.SourceFuncs: field `finalize' is not writable
stack traceback:
    [C]: in field 'field'
    /usr/share/lua/5.3/lgi/record.lua:94: in function </usr/share/lua/5.3/lgi/record.lua:68>
    (...tail calls...)
    [C]: in metamethod '__newindex'
    /usr/share/lua/5.3/lgi/record.lua:129: in function </usr/share/lua/5.3/lgi/record.lua:113>
    (...tail calls...)
    [C]: in ?

Same error happens if you create the SourceFuncs first and try and add the field:

local GLib = require"lgi".GLib
funcs = GLib.SourceFuncs()
funcs.prepare = print

/usr/share/lua/5.3/lgi/record.lua:94: GLib.SourceFuncs: field `prepare' is not writable
stack traceback:
    [C]: in field 'field'
    /usr/share/lua/5.3/lgi/record.lua:94: in function </usr/share/lua/5.3/lgi/record.lua:68>
    (...tail calls...)
    [C]: in metamethod '__newindex'
    stdin:1: in main chunk
    [C]: in ?

Start with record.lua at the reported lines 94 and 129, then reproduce the GLib.SourceFuncs construction and field assignments from the issue. Trace why prepare, check, dispatch, and finalize are rejected as non-writable; done means the reported assignments work without the field error and the existing behavior remains intact.