```typescript
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }
  
  const result = [];
  let count = 1;
  
  for (let i = 0; i < input.length; i++) {
    if (input[i] === input[i + 1]) {
      count++;
    } else {
      result.push(input[i], count.toString());
      count = 1;
    }
    
    // Stop if the run exceeds nine characters
    if (count > 9) {
      throw new Error('Run length exceeds maximum of nine');
    }
  }
  
  return result.join('');
}
```