denoland/deno · error · TypeError

ERR_AMBIGUOUS_ARGUMENT

ERR_AMBIGUOUS_ARGUMENT

Error message

The "error/message" argument is ambiguous. The error message "${actual.message}" is identical to the message.

What it means

Thrown when a string matcher is identical to the message of the caught error. A string matcher never matches anything (it only labels the failure message), so an identical value means the author almost certainly intended message matching; assert refuses with ERR_AMBIGUOUS_ARGUMENT to prevent a test that would silently assert nothing.

Source

Thrown at ext/node/polyfills/assert.ts:394

function expectsError(
  stackStartFn,
  actual,
  error,
  message,
) {
  if (typeof error === "string") {
    if (arguments.length === 4) {
      throw new ERR_INVALID_ARG_TYPE("error", [
        "Object",
        "Error",
        "Function",
        "RegExp",
      ], error);
    }
    if (typeof actual === "object" && actual !== null) {
      if (actual.message === error) {
        throw new ERR_AMBIGUOUS_ARGUMENT(
          "error/message",
          `The error message "${actual.message}" is identical to the message.`,
        );
      }
    } else if (actual === error) {
      throw new ERR_AMBIGUOUS_ARGUMENT(
        "error/message",
        `The error "${actual}" is identical to the message.`,
      );
    }
    message = error;
    error = undefined;
  } else if (
    error != null &&
    typeof error !== "object" &&
    typeof error !== "function"
  ) {
    throw new ERR_INVALID_ARG_TYPE("error", [

View on GitHub (pinned to 89f33cbef2)

Solutions

Use a RegExp matcher: assert.throws(fn, /^boom$/)
Use a constructor plus validation object: assert.throws(fn, { message: /^boom$/ })
Keep the string only if you truly want a failure label, and make its text differ from the error message

Example fix

// before
assert.throws(fn, 'boom');
// after
assert.throws(fn, /^boom$/);

Defensive patterns

Strategy: validation

Validate before calling

if (typeof matcher === 'string') matcher = new RegExp(`^${escapeRegExp(matcher)}$`);
assert.throws(fn, matcher);

Prevention

Use RegExp for message matching; strings only label failures
Escape dynamic strings before embedding them in RegExp

When it happens

Trigger: The function throws new Error('boom') and the test calls assert.throws(fn, 'boom'); the same via assert.rejects(fn, 'boom') when the rejection has message 'boom'.

Common situations: Porting tests from frameworks where a string matcher performs matching; pasting the exact expected message text into the matcher slot.

Related errors

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/f169c2058c4e4319. Report an issue: GitHub.