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

  let result = "";
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (i + 1 < input.length && input[i] === input[i + 1]) {
      count++;
    } else {
      result += input[i] + `${count}`;
      count = 1;
    }
  }

  return result;
}
```