SeleniumHQ/selenium · error · Error

Type must be an instance of String. Received:'${type}'

Error message

Type must be an instance of String. Received:'${type}'

What it means

CaptureScreenshotParameters.imageFormat() validates the first argument (type) is a primitive string (typeof === 'string') before setting it as the image format. This is the format/codec selector (e.g. 'png', 'jpeg'). Passing a number, object, null, undefined, or a String wrapper object fails the typeof check. Note this only validates that type is a string, not that it is a supported format — unsupported formats are caught later server-side (error [15]).

Source

Thrown at javascript/selenium-webdriver/bidi/captureScreenshotParameters.js:62

  origin(origin) {
    if (origin !== Origin.VIEWPORT && origin !== Origin.DOCUMENT) {
      throw new Error(`Origin must be one of ${Object.values(Origin)}. Received:'${origin}'`)
    }
    this.#map.set('origin', origin)
    return this
  }

  /**
   * Sets the image format and quality for capturing a screenshot.
   *
   * @param {string} type - The image format type.
   * @param {number} [quality] - The image quality (optional).
   * @throws {Error} If the type is not a string or if the quality is not a number.
   * @returns {CaptureScreenshotParameters} - The current instance of the CaptureScreenshotParameters for chaining.
   */
  imageFormat(type, quality = undefined) {
    if (typeof type !== 'string') {
      throw new Error(`Type must be an instance of String. Received:'${type}'`)
    }

    this.#map.set('type', type)

    if (quality !== undefined) {
      if (typeof quality !== 'number') {
        throw new Error(`Quality must be a number. Received:'${quality}'`)
      }
      this.#map.set('quality', quality)
    }
    return this
  }

  /**
   * Sets the clip rectangle for capturing a screenshot.
   *
   * @param {BoxClipRectangle|ElementClipRectangle} clipRectangle - The clip rectangle to set.
   * @throws {Error} If the clipRectangle is not an instance of ClipRectangle.

View on GitHub (pinned to aa36b38e69)

Solutions

1. Pass a primitive string format as the first argument: imageFormat('png') or imageFormat('jpeg', 80).
2. If you only want to set quality, you must still supply a type string first.
3. Use lowercase canonical format names ('png', 'jpeg').
4. Validate config values are strings before forwarding.

Example fix

// before
params.imageFormat(80) // number where type string expected

// after
params.imageFormat('jpeg', 80)

if (typeof type !== 'string') {
  throw new TypeError('imageFormat type must be a string (e.g. png, jpeg)')
}

function isImageFormatType(t) {
  return typeof t === 'string'
}

When it happens

Trigger: Calling imageFormat(undefined) or imageFormat(null) for the type. Passing a numeric or object value. Passing a String object wrapper (typeof 'object'). Forgetting the type argument when also wanting to set quality.

Common situations: Calling imageFormat(quality) positionally (passing quality where type belongs). Config code forwarding a non-string. Building the call dynamically and omitting the type.

Related errors

• Quality must be a number. Received:'${quality}'
• Origin must be one of ${Object.values(Origin)}. Received:'${
• ClipRectangle must be an instance of ClipRectangle. Received
• Http method must be a string. Received: '${method})'
• Url must be a string. Received:'${url}'

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/a237bd11f7047b18. Report an issue: GitHub.