```ts
function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

  let encoded = '';
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (input[i] === input[i + 1]) {
      count++;
    } else {
      encoded += `${input[i]}${count > 9 ? '9' : count}`;
      count = 1;
    }
  }

  return encoded || '';
}
```