SeleniumHQ/selenium · error · Error

Value must be an instance of BytesValue. Received: '${value}

Error message

Value must be an instance of BytesValue. Received: '${value}'

What it means

Thrown by the Header constructor (networkTypes.js) when its value argument is not an instance of BytesValue. Per the WebDriver BiDi spec a network Header value must be a BytesValue (type+value) object, so the constructor enforces the type before storing it. A raw string or number is rejected because Header.asMap() later calls value.asMap().

Source

Thrown at javascript/selenium-webdriver/bidi/networkTypes.js:102

    return map
  }
}

/**
 * Represents a header with a name and value.
 * Described in https://w3c.github.io/webdriver-bidi/#type-network-Header.
 */
class Header {
  /**
   * Creates a new Header instance.
   * @param {string} name - The name of the header.
   * @param {BytesValue} value - The value of the header.
   * @throws {Error} If the value is not an instance of BytesValue.
   */
  constructor(name, value) {
    this._name = name
    if (!(value instanceof BytesValue)) {
      throw new Error(`Value must be an instance of BytesValue. Received: '${value}'`)
    }
    this._value = value
  }

  /**
   * Gets the name of the header.
   * @returns {string} The name of the header.
   */
  get name() {
    return this._name
  }

  /**
   * Gets the value of the header.
   * @returns {BytesValue} The value of the header.
   */
  get value() {
    return this._value

View on GitHub (pinned to aa36b38e69)

Solutions

1. Wrap the value in a BytesValue instance before constructing the Header: new Header('content-type', new BytesValue(BytesValue.Type.STRING, 'application/json')).
2. For binary data, use new BytesValue(BytesValue.Type.BASE64, base64String).
3. If you receive header data as plain objects, map each to new Header(h.name, new BytesValue(h.value.type, h.value.value)).

Example fix

// before
const header = new Header('content-type', 'application/json')

// after
const { BytesValue } = require('selenium-webdriver/bidi/networkTypes')
const header = new Header('content-type', new BytesValue(BytesValue.Type.STRING, 'application/json'))

function toHeaderValue(v) {
  if (v instanceof BytesValue) return v
  if (typeof v === 'string') return new BytesValue(BytesValue.Type.STRING, v)
  throw new TypeError('Header value must be BytesValue or string')
}

function isBytesValue(v) {
  return v instanceof BytesValue
}

When it happens

Trigger: Calling new Header('content-type', 'application/json') (raw string), new Header('x', 123), or new Header('x', {type:'string', value:'v'}) (plain object that is structurally but not structurally an instance).

Common situations: Assuming Header takes a plain string value (as in classic HTTP libraries); deserializing JSON into a plain object and passing it; forgetting to wrap the value in BytesValue.

Related errors

• Value must be an instance of BytesValue. Received: ${typeof
• CookieHeader must be an instance of Header. Received:'${head
• Header must be an instance of Header. Received:'${header}'
• Params must be an instance of AddInterceptParameters. Receiv
• Params must be an instance of ContinueRequestParameters. Rec

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/923dde4d56822a5e. Report an issue: GitHub.