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

  let result = '';
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (i > 0 && input[i] === input[i - 1]) {
      count++;
    } else {
      const charCode = input.charCodeAt(i);
      result += String.fromCharCode(charCode, count);
      count = 1;
    }
  }

  return result;
}
```