```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') throw new Error('Input must be a string');
  if (input.length === 0) return '';

  let result = '';
  let count = 1;

  for (let i = 1; i <= input.length; i++) {
    const char = input[i - 1];
    const nextChar = input[i] ?? ''; // Handle the case where we're at the end of the string

    if (char === nextChar) {
      count++;
    } else {
      result += char + (count > 1 ? count.toString() : '');
      count = 1;
    }
  }

  return result;
}
```