remotion-dev/remotion · error · TypeError

Cannot interpolate font weight

Error message

Cannot interpolate font weight "${output}". Expected "normal", "bold", or a number between 1 and 1000

What it means

interpolate() can animate CSS font-weight values, which must be 'normal' (400), 'bold' (700), or a number 1–1000. If a font-weight string/number in the output range cannot be parsed to a valid weight, a TypeError is thrown listing the accepted formats.

Solutions

1. Use numeric weights 1–1000 (e.g. '400', '700') or exactly 'normal'/'bold'.
2. Fix out-of-range numbers to be within 1–1000.
3. Interpolate the number 400→700 and apply with fontWeight: String(w).

Example fix

// before
interpolate(f, [0,1], ['thin','semibold'])
// after
interpolate(f, [0,1], ['100','600'])

const isValidWeight = (v: string | number) =>
  v === 'normal' || v === 'bold' ||
  (typeof v === 'string' && /^\d+$/.test(v) && +v >= 1 && +v <= 1000) ||
  (typeof v === 'number' && v >= 1 && v <= 1000);

const isFontWeight = (v: unknown): v is number | 'normal' | 'bold' =>
  v === 'normal' || v === 'bold' ||
  (typeof v === 'number' && v >= 1 && v <= 1000);

try {
  interpolate(f, r, weights, {outputType: 'font-weight'});
} catch (e) {
  if (e instanceof TypeError) useNumericWeights();
}

When it happens

Trigger: Output range containing strings like '900px', 'thin', '1000.5', or numbers < 1 or > 1000 where font-weight values are expected.

Common situations: Typo'd weights ('500px'); using named weights like 'semibold' which aren't CSS-standard; passing rem/px-styled values into font-weight interpolation.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• bezier x values must be in [0, 1] range
• Cannot interpolate " " as because it is a rotate value
• Cannot interpolate " " as because it is a transform-origin…
• A "duration" of a spring is NaN, which it must not be
• A "duration" of a spring must be a "number" but is

AI-assisted analysis of remotion-dev/remotion@46a3a6bf13 (2026-09-18). Data as JSON: /api/errors/ddf047df3403b262. Report an issue: GitHub.

Appendix: source

Thrown at packages/core/src/interpolate.ts:976

			if (lowerCaseOutput === 'normal') {
				return 400;
			}

			if (lowerCaseOutput === 'bold') {
				return 700;
			}
		}

		const match =
			typeof output === 'string' ? cssNumberRegex.exec(output) : null;
		const value =
			typeof output === 'number'
				? output
				: match !== null && match[2] === undefined
					? Number(match[1])
					: Number.NaN;
		if (!Number.isFinite(value) || value < 1 || value > 1000) {
			throw new TypeError(
				`Cannot interpolate font weight "${output}". Expected "normal", "bold", or a number between 1 and 1000`,
			);
		}

		return value;
	});

	return interpolateNumber({
		input,
		inputRange,
		outputRange: normalizedOutputRange,
		options,
	});
};

const interpolateDiscreteString = ({
	input,
	inputRange,

View on GitHub (pinned to 46a3a6bf13)