dotnet/runtime · error · Error

event must be an object, but got ${JSON.stringify(event)}

Error message

event must be an object, but got ${JSON.stringify(event)}

What it means

Thrown by mono_wasm_raise_debug_event() in debug.ts when the `event` argument is not of type 'object'. This function raises a debug event (e.g. breakpoint/script loaded) to the BrowserDebugProxy which listens on console.debug with a fixed magic GUID. Passing a primitive (string, number, boolean, symbol, bigint, undefined) instead of an event object is a programming contract violation. Note: typeof null === 'object', so null passes this check but will throw a TypeError on the next line accessing event.eventName.

Source

Thrown at src/mono/browser/runtime/debug.ts:132

    forceThreadMemoryViewRefresh();
}

export function mono_wasm_detach_debugger (): void {
    forceThreadMemoryViewRefresh();
    cwraps.mono_wasm_set_is_debugger_attached(false);
}

export function mono_wasm_change_debugger_log_level (level: number): void {
    forceThreadMemoryViewRefresh();
    cwraps.mono_wasm_change_debugger_log_level(level);
}

/**
 * Raises an event for the debug proxy
 */
export function mono_wasm_raise_debug_event (event: WasmEvent, args = {}): void {
    if (typeof event !== "object")
        throw new Error(`event must be an object, but got ${JSON.stringify(event)}`);

    if (event.eventName === undefined)
        throw new Error(`event.eventName is a required parameter, in event: ${JSON.stringify(event)}`);

    if (typeof args !== "object")
        throw new Error(`args must be an object, but got ${JSON.stringify(args)}`);

    // eslint-disable-next-line no-console
    console.debug("mono_wasm_debug_event_raised:aef14bca-5519-4dfe-b35a-f867abc123ae", JSON.stringify(event), JSON.stringify(args));
}

export function mono_wasm_wait_for_debugger (): Promise<void> {
    return new Promise<void>((resolve) => {
        const interval = setInterval(() => {
            if (runtimeHelpers.waitForDebugger != 1) {
                return;
            }
            clearInterval(interval);

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Pass an object literal: { eventName: '...', ...payload }.
2. Ensure the variable you pass is not undefined (a missing import/typo yields undefined).
3. If you intended no event, do not call the function rather than passing a primitive.

Example fix

// before
mono_wasm_raise_debug_event('breakpointHit');

// after
mono_wasm_raise_debug_event({ eventName: 'breakpointHit', id: bpId });

function raiseDebugEventSafe(event: unknown, args?: unknown) {
  if (event === null || typeof event !== "object") {
    throw new TypeError('event must be a non-null object');
  }
  // ...then call mono_wasm_raise_debug_event(event as WasmEvent, args as any);
}

function isWasmEvent(event: unknown): event is { eventName: string } {
  return event !== null && typeof event === "object"
    && typeof (event as any).eventName === "string";
}

null

When it happens

Trigger: Calling mono_wasm_raise_debug_event with a string/number/undefined instead of an object, e.g. mono_wasm_raise_debug_event('breakpoint') or mono_wasm_raise_debug_event(someId). Internal callers that build the event lazily and pass a primitive placeholder.

Common situations: Hand-written instrumentation or a fork that calls mono_wasm_raise_debug_event with a non-object. A refactor that changed what is passed in but did not wrap it in { eventName, ... }.

Related errors

• args must be an object, but got ${JSON.stringify(args)}
• event.eventName is a required parameter, in event: ${JSON.st
• "arguments" should be an array, but was ${request.arguments}
• Failed on mono_wasm_send_dbg_command_with_parms
• Failed on mono_wasm_send_dbg_command

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/213466d04d2cc81d. Report an issue: GitHub.