n8n-io/n8n · error · ExpressionExtensionError

can't be used on value

Error message

${functionName} can't be used on ${String(value)} value

What it means

checkIfValueDefinedOrThrow fired inside extend(): the receiver of the expression function is null or undefined, so the function cannot run on it. The message names the function and the actual value, making the null-propagation visible in the expression editor.

Solutions

1. Guard the chain: use optional chaining or a default before calling the function (e.g. ($json.field ?? '').toSnakeCase())
2. Fix the upstream step so the field is populated rather than null
3. Check for typos in the field/property name — a wrong key resolves to undefined

When it happens

Trigger: Thrown at packages/@n8n/expression-runtime/src/extensions/utils.ts:25 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/c9ae90b965d905c6. Report an issue: GitHub.

Appendix: source

Thrown at packages/@n8n/expression-runtime/src/extensions/utils.ts:25

export const convertToDateTime = (value: string | Date | DateTime): DateTime | undefined => {
	let converted: DateTime | undefined;

	if (typeof value === 'string') {
		converted = DateTime.fromJSDate(new Date(value));
		if (converted.invalidReason !== null) {
			return;
		}
	} else if (value instanceof Date) {
		converted = DateTime.fromJSDate(value);
	} else if (DateTime.isDateTime(value)) {
		converted = value;
	}
	return converted;
};

export function checkIfValueDefinedOrThrow<T>(value: T, functionName: string): void {
	if (value === undefined || value === null) {
		throw new ExpressionExtensionError(`${functionName} can't be used on ${String(value)} value`, {
			description: `To ignore this error, add a ? to the variable before this function, e.g. my_var?.${functionName}`,
		});
	}
}

View on GitHub (pinned to 5ac6606e81)