avajs/ava · error · AssertionError

The `instanceOf` property of the second argument to `${asser

Error message

The `instanceOf` property of the second argument to `${assertion}` must be a function

What it means

validateExpectations guard: the expectations object contained an instanceOf key whose value is not a constructor function (e.g. a string class name or an instance). The input at fault is expectations.instanceOf.

Source

Thrown at lib/assert.js:99

			});
		}

		expectations = {};
	} else if (
		typeof expectations === 'function'
		|| typeof expectations === 'string'
		|| expectations instanceof RegExp
		|| typeof expectations !== 'object'
		|| Array.isArray(expectations)
		|| Object.keys(expectations).length === 0
	) {
		throw new AssertionError(`The second argument to \`${assertion}\` must be an expectation object, \`null\` or \`undefined\``, {
			assertion,
			formattedDetails: [formatWithLabel('Called with:', expectations)],
		});
	} else {
		if (Object.hasOwn(expectations, 'instanceOf') && typeof expectations.instanceOf !== 'function') {
			throw new AssertionError(`The \`instanceOf\` property of the second argument to \`${assertion}\` must be a function`, {
				assertion,
				formattedDetails: [formatWithLabel('Called with:', expectations)],
			});
		}

		if (
			Object.hasOwn(expectations, 'message')
			&& typeof expectations.message !== 'string'
			&& !(expectations.message instanceof RegExp)
			&& !(typeof expectations.message === 'function')
		) {
			throw new AssertionError(`The \`message\` property of the second argument to \`${assertion}\` must be a string, regular expression or a function`, {
				assertion,
				formattedDetails: [formatWithLabel('Called with:', expectations)],
			});
		}

		if (Object.hasOwn(expectations, 'name') && typeof expectations.name !== 'string') {

View on GitHub (pinned to bbfd946322)

Solutions

1. Pass the constructor itself: {instanceOf: TypeError}, not 'TypeError'
2. Import the error class you expect and reference it directly
3. Remove instanceOf if you meant to match with message or name

When it happens

Trigger: Thrown at lib/assert.js:99 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of avajs/ava@bbfd946322 (2026-09-02). Data as JSON: /api/errors/42370c467716884c. Report an issue: GitHub.