File ecosystem.config.js malformated

Using this simple config in my ecosystem.config.js :

module.exports = {
apps : [{
script: 'app.js',
watch: '.'
}]
};

I have the following error :

[PM2][ERROR] File ecosystem.config.js malformated
TypeError: Cannot convert undefined or null to object
at Common.parseConfig (/Users/louisbrulenaudet/.yarn/berry/cache/pm2-npm-5.3.0-910c0e1105-9.zip/node_modules/pm2/lib/Common.js:316:26)
at API._startJson (/Users/louisbrulenaudet/.yarn/berry/cache/pm2-npm-5.3.0-910c0e1105-9.zip/node_modules/pm2/lib/API.js:934:25)
at API.start (/Users/louisbrulenaudet/.yarn/berry/cache/pm2-npm-5.3.0-910c0e1105-9.zip/node_modules/pm2/lib/API.js:329:12)
at /Users/louisbrulenaudet/.yarn/berry/cache/pm2-npm-5.3.0-910c0e1105-9.zip/node_modules/pm2/lib/binaries/CLI.js:290:13
at /Users/louisbrulenaudet/.yarn/berry/cache/async-npm-3.2.4-aba13508f9-9.zip/node_modules/async/internal/withoutIndex.js:8:40
at replenish (/Users/louisbrulenaudet/.yarn/berry/cache/async-npm-3.2.4-aba13508f9-9.zip/node_modules/async/internal/eachOfLimit.js:81:17)
at /Users/louisbrulenaudet/.yarn/berry/cache/async-npm-3.2.4-aba13508f9-9.zip/node_modules/async/internal/eachOfLimit.js:86:9
at eachLimit (/Users/louisbrulenaudet/.yarn/berry/cache/async-npm-3.2.4-aba13508f9-9.zip/node_modules/async/forEachLimit.js:47:43)
at awaitable (/Users/louisbrulenaudet/.yarn/berry/cache/async-npm-3.2.4-aba13508f9-9.zip/node_modules/async/internal/awaitify.js:13:28)
at Command. (/Users/louisbrulenaudet/.yarn/berry/cache/pm2-npm-5.3.0-910c0e1105-9.zip/node_modules/pm2/lib/binaries/CLI.js:289:7)

I use Node 20.6.0 with Yarn berry.

Start with the failing configuration in ecosystem.config.js and the stack trace location in lib/Common.js:316, then trace how API.js and the CLI invoke config parsing. Reproduce the malformed-file error with the supplied Node 20 and Yarn Berry setup; done means this valid configuration is accepted without the reported TypeError.