```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++) {
    const currentChar = input[i];
    const nextChar = input[i + 1] || '';

    if (currentChar === nextChar) {
      count++;
    } else {
      encoded += `${currentChar}${count > 1 ? count : ''}`;
      count = 1;
    }
  }

  return encoded;
}
```