```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 = 0; i < input.length; i++) {
        const char = input[i];
        const nextChar = input[i + 1] || '';

        if (char === nextChar) {
            count++;
        } else {
            encoded += char + (count > 1 ? count : '');
            count = 1;
        }
    }

    return encoded.length > 9 ? 'Encodings longer than nine characters are not allowed' : encoded;
}
```