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

  if (input.length === 0) return '';

  let encoded = '';
  let count = 1;

  for (let i = 1; i <= input.length; i++) {
    if (i === input.length || input[i] !== input[i - 1]) {
      encoded += `${input[i - 1]}${count}`;
      count = 1;
    } else {
      count++;
    }
  }

  return encoded;
}
```