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

    let encoded = '';
    let count = 1;

    for (let i = 1; i <= input.length; i++) {
        if (i < input.length && input[i] === input[i - 1]) {
            count++;
            continue;
        }
        encoded += `${input[i - 1]}${count > 9 ? `9+` : count}`;
        count = 1;
    }

    return encoded;
}
```