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

  const encoded = [];
  let count = 1;

  for (let i = 1; i <= input.length; i++) {
    if (input[i] === input[i - 1]) {
      count++;
    } else {
      encoded.push(input[i - 1] + count.toString());
      count = 1;
    }
  }

  return encoded.join('');
}
```