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

    if (input === '') {
        return '';
    }

    let encoded = '';
    let count = 1;

    for (let i = 0; i < input.length; i++) {
        if (i < input.length - 1 && input[i] === input[i + 1]) {
            count++;
        } else {
            encoded += input[i] + (count > 1 ? count : '');
            count = 1;
        }
    }

    return encoded;
}
```