microsoft/playwright · error · Error

Expected value of header "${key}" to be String, but "${typeo

Error message

Expected value of header "${key}" to be String, but "${typeof value}" is found.

What it means

Error "Expected value of header "${key}" to be String, but "${typeof value}" is found." thrown in microsoft/playwright.

Source

Thrown at packages/playwright-core/src/client/network.ts:827

      const waiter = Waiter.createForEvent(this, event);
      waiter.rejectOnTimeout(timeoutOptions, `Timeout ${timeoutOptions.timeout}ms exceeded while waiting for event "${event}"`);
      if (event !== Events.WebSocket.Error)
        waiter.rejectOnEvent(this, Events.WebSocket.Error, new Error('Socket error'));
      if (event !== Events.WebSocket.Close)
        waiter.rejectOnEvent(this, Events.WebSocket.Close, new Error('Socket closed'));
      waiter.rejectOnEvent(this._page, Events.Page.Close, () => this._page._closeErrorWithReason());
      const result = await waiter.waitForEvent(this, event, predicate as any);
      waiter.dispose();
      return result;
    });
  }
}

export function validateHeaders(headers: Headers) {
  for (const key of Object.keys(headers)) {
    const value = headers[key];
    if (!Object.is(value, undefined) && !isString(value))
      throw new Error(`Expected value of header "${key}" to be String, but "${typeof value}" is found.`);
  }
}

export class RouteHandler {
  private handledCount = 0;
  private readonly _baseURL: string | undefined;
  private readonly _times: number;
  readonly url: URLMatch;
  readonly handler: RouteHandlerCallback;
  private _ignoreException: boolean = false;
  private _activeInvocations: Set<{ complete: Promise<void>, route: Route }> = new Set();
  private _savedZone: Zone;

  constructor(baseURL: string | undefined, url: URLMatch, handler: RouteHandlerCallback, times: number = Number.MAX_SAFE_INTEGER) {
    this._baseURL = baseURL;
    this._times = times;
    this.url = url;
    this.handler = handler;

View on GitHub (pinned to c8fc3bf8d3)

When it happens

Trigger: Thrown at packages/playwright-core/src/client/network.ts:827 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of microsoft/playwright@c8fc3bf8d3 (2026-08-12). Data as JSON: /api/errors/c708dc7b8600fa3f. Report an issue: GitHub.