Error when using configuration (The "file" argument must be of type string)

The error

When I open an Erlang file, I get the following error:

[coc.nvim] error:     at Object.<anonymous> (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc-erlang_ls/lib/index.js:156:20)

:CocInfo shows the following error:

## versions

vim version: VIM - Vi IMproved 8.1 (2018 May 18, compiled Sep 25 2018 13:59:45)
node version: v13.5.0
coc.nvim version: 0.0.74-cd348f4905
term: dumb
platform: linux

## Messages
Messages maintainer: Bram Moolenaar <Bram@vim.org>
[coc.nvim] error: UnhandledRejection: The "file" argument must be of type string. Received type object
[coc.nvim] error: TypeError [ERR_INVALID_ARG_TYPE]: The "file" argument must be of type string. Received type object
[coc.nvim] error:     at validateString (internal/validators.js:118:11)
[coc.nvim] error:     at normalizeSpawnArguments (child_process.js:405:3)
[coc.nvim] error:     at spawn (child_process.js:545:13)
[coc.nvim] error:     at Object.execFile (child_process.js:231:17)
[coc.nvim] error:     at /home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc-erlang_ls/lib/index.js:102:25
[coc.nvim] error:     at LanguageClient.createMessageTransports (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:54119:48)
[coc.nvim] error:     at LanguageClient.createConnection (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:56580:21)
[coc.nvim] error:     at LanguageClient.resolveConnection (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:56362:44)
[coc.nvim] error:     at LanguageClient.start (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:56299:14)
[coc.nvim] error:     at Object.<anonymous> (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc-erlang_ls/lib/index.js:154:20)
## Output channel: coc-erlang

## Output channel: erlang_ls

[Error  - 9:30:35 PM] Starting client failed:
TypeError [ERR_INVALID_ARG_TYPE]: The "file" argument must be of type string. Received type object
    at validateString (internal/validators.js:118:11)
    at normalizeSpawnArguments (child_process.js:405:3)
    at spawn (child_process.js:545:13)
    at Object.execFile (child_process.js:231:17)
    at /home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc-erlang_ls/lib/index.js:102:25
    at LanguageClient.createMessageTransports (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:54119:48)
    at LanguageClient.createConnection (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:56580:21)
    at LanguageClient.resolveConnection (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:56362:44)
    at LanguageClient.start (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc.nvim/build/index.js:56299:14)
    at Object.<anonymous> (/home/hcs/d/cp/vim/vimfiles/pack/dixon-plugins/opt/coc-erlang_ls/lib/index.js:154:20)

The possible reason

Here is my configuration:

{
        "erlang_ls.erlang_ls_path": {
          "type": "string",
          "default": "/home/hcs/w/els/erlang_ls/_build/default/bin/erlang_ls",
          "description": "path of erlang_ls"
        },
        "erlang_ls.port": {
          "type": "number",
          "default": 19527,
          "description": "Port to communicate with language server."
        },
        "erlang_ls.trace.server": {
          "type": "string",
          "default": "off",
          "enum": [
            "off",
            "messages",
            "verbose"
          ],
          "description": "Trace level of vim language server"
        }
}

If I put the following debug printout lines into coc-erlang_ls/lib/index.js (ts source):

 function startServer(serverPath, serverPort) {
     return function () {
+        logger.appendLine('serverPath=');
+        logger.appendLine(JSON.stringify(serverPath));
         child_process_1.execFile(serverPath, [serverPort.toString()]);

then it shows that serverPath is an object (not a string):

serverPath=
{"type":"string","default":"/home/hcs/w/els/erlang_ls/_build/default/bin/erlang_ls","description":"path of erlang_ls"}

The problem is that the code that eventually tries to use the serverPath variable expects a string (not an object).

If I modify the configuration file to have a string instead of an object (not only for erlang_ls_path but also for port):

{
        "erlang_ls.erlang_ls_path": "/home/hcs/w/els/erlang_ls/_build/default/bin/erlang_ls",
        "erlang_ls.port": 19527,
        "erlang_ls.trace.server": {
          "type": "string",
          "default": "off",
          "enum": [
            "off",
            "messages",
            "verbose"
          ],
          "description": "Trace level of vim language server"
        }
}

then coc-erlang_ls can start erlang_ls as expected and the problem goes away.

The question is: why? Any ideas about why configuration is apparently read differently on my machine?

This is the line that reads the configuration, but I didn't get deeper into what kind of magic is done by this WorkspaceConfiguration object.

hyhugh / coc-erlang_ls

Error when using configuration (The "file" argument must be of type string) #2

The error

The possible reason