```ts
/**
 * Encodes a string using run-length encoding.
 * @param input - The string to encode.
 * @returns The encoded string.
 * @throws Error if the input is not a string.
 */
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 currentChar = input[0];
  let count = 1;

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

  encoded += `${currentChar}${count}`;

  return encoded;
}
```