remotion-dev/remotion · error · TypeError

You passed in a a function to the volume prop but it did not

Error message

You passed in a a function to the volume prop but it did not return a number but a value of type ${typeof evaluated} for frame ${frame}

What it means

TypeError thrown by `evaluateVolume` when the `volume` prop is a function but calling `volume(frame)` produced a value whose type is not 'number' (after multiplying by mediaVolume). Remotion's VolumeProp accepts either a number or a function (frame) => number; if the function returns a string, object, undefined, etc., this error fires. Note the original message contains a duplicated 'a a' typo. The evaluated value's typeof is included along with the frame number.

Source

Thrown at packages/core/src/volume-prop.ts:22

	frame,
	volume,
	mediaVolume = 1,
}: {
	frame: number;
	volume: VolumeProp | undefined;
	mediaVolume: number;
}): number => {
	if (typeof volume === 'number') {
		return volume * mediaVolume;
	}

	if (typeof volume === 'undefined') {
		return Number(mediaVolume);
	}

	const evaluated = volume(frame) * mediaVolume;
	if (typeof evaluated !== 'number') {
		throw new TypeError(
			`You passed in a a function to the volume prop but it did not return a number but a value of type ${typeof evaluated} for frame ${frame}`,
		);
	}

	if (Number.isNaN(evaluated)) {
		throw new TypeError(
			`You passed in a function to the volume prop but it returned NaN for frame ${frame}.`,
		);
	}

	if (!Number.isFinite(evaluated)) {
		throw new TypeError(
			`You passed in a function to the volume prop but it returned a non-finite number for frame ${frame}.`,
		);
	}

	return Math.max(0, evaluated);
};

View on GitHub (pinned to 78fe4bb3fd)

Solutions

1. Ensure the volume callback always returns a number for every frame: add explicit `return 0;` in fallback branches.
2. Annotate the callback as `(frame: number): number => ...` so TypeScript catches non-number returns at compile time.
3. Unit-test the callback across all frames in the composition duration to find the offending frame noted in the message.

Example fix

// before
<Video src={s} volume={(f) => {
  if (f > 30) return 0.5;
  // missing return -> undefined
}} />

// after
<Video src={s} volume={(f: number): number => {
  if (f > 30) return 0.5;
  return 0;
}} />

import type {VolumeProp} from 'remotion';

const validateVolumeFn = (v: VolumeProp, frames: number[]) => {
  if (typeof v !== 'function') return;
  for (const f of frames) {
    const out = v(f);
    if (typeof out !== 'number')
      throw new Error(`volume() returned ${typeof out} at frame ${f}`);
  }
};

const isVolumeFn = (v: unknown): v is ((frame: number) => number) =>
  typeof v === 'function';

When it happens

Trigger: Passing `volume={(frame) => someString}` or any volume callback whose return is not a number for at least one frame in the composition's range. For example returning `undefined` from a branch, a string like '0.5', or forgetting to return from the callback.

Common situations: A volume callback with a missing `return` in one branch; returning a value from a map/lookup that is occasionally undefined; migrating from a number volume to a function and leaving a non-number return type.

Related errors

• The `<Html5Audio>` tag requires a string for `src`, but got
• You have passed a volume of type ${typeof props.volume} to y
• You have passed a volume below 0 to your <${component} /> co
• You passed in a function to the volume prop but it returned
• You passed in a function to the volume prop but it returned

AI-assisted analysis of remotion-dev/remotion@78fe4bb3fd (2026-08-12). Data as JSON: /api/errors/0fff1bdce4600f9d. Report an issue: GitHub.