remix-run/remix · error · TypeError

ERR_INVALID_RETURN_VALUE

ERR_INVALID_RETURN_VALUE

Error message

Expected instance of Promise to be returned from the "promiseFn" function but got type ${typeof promise} (${stringify(promise)}).

What it means

When assert.promise() receives a function, it calls it and requires the return value to be a genuine Promise instance. If the function returns a thenable-less value (or a custom thenable that isn't a Promise), it throws ERR_INVALID_RETURN_VALUE.

Source

Thrown at packages/assert/src/lib/assert.ts:614

          actual: e,
          expected: expectedError,
          operator: 'doesNotThrow',
          generatedMessage: false,
        },
        false,
      )
    }

    throw e
  }
}

function getPromise(value: (() => Promise<unknown>) | Promise<unknown>): Promise<unknown> {
  if (typeof value === 'function') {
    let promise = value()

    if (!(promise instanceof Promise)) {
      throw createNodeTypeError(
        'ERR_INVALID_RETURN_VALUE',
        `Expected instance of Promise to be returned from the "promiseFn" function but got type ${typeof promise} (${stringify(
          promise,
        )}).`,
      )
    }

    return promise
  }

  if (!(value instanceof Promise)) {
    throw createInvalidArgumentTypeError(
      'promiseFn',
      'of type function or an instance of Promise',
      value,
    )
  }

View on GitHub (pinned to 9696913134)

Solutions

1. Ensure the function returns a native Promise (async functions or Promise.resolve(...))
2. Await or wrap third-party thenables: Promise.resolve(thenable)

Example fix

// before
assert.promise(() => fetchLater()) // returns undefined
// after
assert.promise(() => Promise.resolve(fetchLater()))

let result = promiseFn()
if (!(result instanceof Promise)) result = Promise.resolve(result)

const returnsNativePromise = (fn: () => unknown): fn is () => Promise<unknown> =>
  fn() instanceof Promise

When it happens

Trigger: assert.throws-like usage where promiseFn returns undefined, a plain value, or an object with a .then that is not a native Promise.

Common situations: Async functions transpiled/monkey-patched to return non-native thenables in test environments, or forgetting to return the promise inside the function.

Related errors

• ERR_INVALID_ARG_TYPE
• ERR_INVALID_ARG_VALUE

AI-assisted analysis of remix-run/remix@9696913134 (2026-08-27). Data as JSON: /api/errors/78d618249ba6888e. Report an issue: GitHub.