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

    const result = [];
    let count = 1;

    for (let i = 1; i <= input.length; i++) {
        if (i === input.length || input[i] !== input[i - 1]) {
            result.push(input[i - 1], count.toString());
            count = 1;
        } else {
            count++;
        }
    }

    return result.join('');
}
```