nodejs/node · error · InvalidArgumentError

UND_ERR_INVALID_ARG

UND_ERR_INVALID_ARG

Error message

Invalid lookup. Must be a function

What it means

Thrown by the DNS interceptor factory (`InvalidArgumentError`, code `UND_ERR_INVALID_ARG`) when `lookup` is provided and is not a function. `lookup` lets you plug in a custom resolver (e.g. `dns.lookup`, a multi-resolver strategy, or a service-discovery hook) instead of the default Node resolver. The interceptor calls it per hostname to obtain address records, so anything non-callable would fail at runtime inside the dispatch path.

Source

Thrown at deps/undici/src/lib/interceptor/dns.js:495

    interceptorOpts?.affinity != null &&
    interceptorOpts?.affinity !== 4 &&
    interceptorOpts?.affinity !== 6
  ) {
    throw new InvalidArgumentError('Invalid affinity. Must be either 4 or 6')
  }

  if (
    interceptorOpts?.dualStack != null &&
    typeof interceptorOpts?.dualStack !== 'boolean'
  ) {
    throw new InvalidArgumentError('Invalid dualStack. Must be a boolean')
  }

  if (
    interceptorOpts?.lookup != null &&
    typeof interceptorOpts?.lookup !== 'function'
  ) {
    throw new InvalidArgumentError('Invalid lookup. Must be a function')
  }

  if (
    interceptorOpts?.pick != null &&
    typeof interceptorOpts?.pick !== 'function'
  ) {
    throw new InvalidArgumentError('Invalid pick. Must be a function')
  }

  if (
    interceptorOpts?.storage != null &&
    (typeof interceptorOpts?.storage?.get !== 'function' ||
      typeof interceptorOpts?.storage?.set !== 'function' ||
      typeof interceptorOpts?.storage?.full !== 'function' ||
      typeof interceptorOpts?.storage?.delete !== 'function'
    )
  ) {
    throw new InvalidArgumentError('Invalid storage. Must be a object with methods: { get, set, full, delete }')

View on GitHub (pinned to 1b2de5e052)

Solutions

1. Pass a function reference, e.g. `lookup: dns.lookup` after `const dns = require('node:dns')`, or `lookup: myCustomResolver`.
2. Make sure you import the function, not the module: pass `dns.lookup`, not `dns`.
3. If using a service-discovery client, pass its resolve callback directly, not a config object wrapping it.

Example fix

// before
const dns = require('node:dns')
interceptor dns({ lookup: 'dns.lookup' })
dns({ lookup: { resolve: customResolve } })

// after
dns({ lookup: dns.lookup })
dns({ lookup: customResolve })

function validateLookup(v) {
  if (v == null) return undefined
  if (typeof v !== 'function') throw new Error('lookup must be a function')
  return v
}

function isFunction(v) {
  return typeof v === 'function'
}

When it happens

Trigger: Passing `lookup: 'dns.lookup'` (a string name), an object `{ resolve: fn }`, a hostname string, or a value that is not callable. The guard is `lookup != null && typeof !== 'function'`.

Common situations: Referencing `dns.lookup` without importing `node:dns`; passing a bound method's name as a string; wrapping the resolver in an object (e.g. `{ lookup: realLookup }`) and passing the wrapper instead of the function. Default behavior uses Node's built-in resolver, so omit unless you need customization.

Related errors

• Invalid maxTTL. Must be a positive number
• Invalid maxItems. Must be a positive number and greater than
• Invalid affinity. Must be either 4 or 6
• Invalid dualStack. Must be a boolean
• expected opts.methods to be an array, got ${typeof methods}

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/efe484bd26e22e50. Report an issue: GitHub.