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

  if (input === '') {
    return '';
  }

  let result = '';
  let currentChar = input.charAt(0);
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    if (input.charAt(i) === currentChar) {
      count++;
    } else {
      result += currentChar + String(count);
      currentChar = input.charAt(i);
      count = 1;
    }
  }
  
  result += currentChar + String(count);

  return result;
}
```