```ts
/**
 * Encodes input string using run-length encoding.
 * Throws Error if input is not a string.
 *
 * @param input - The string to encode.
 * @returns The run-length encoded string.
 */
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 - 1; i++) {
    if (input[i] === input[i + 1]) {
      count++;
    } else {
      result += `${input[i]}${count}`;
      count = 1;
    }
  }

  result += `${input[input.length - 1]}${count}`;

  return result;
}
```