```typescript
/**
 * Returns the run-length encoded string representation of input.
 * Throws Error if input is not a string.
 */
export function runLengthEncode(input: string): string {
    if (typeof input !== 'string') {
        throw new Error('Input must be a string');
    }

    const result = [];
    let currentChar = input[0];
    let charCount = 1;

    for (let i = 1; i <= input.length; i++) {
        if (i < input.length && input[i] !== currentChar) {
            result.push(`${currentChar}${charCount}`);
            currentChar = input[i];
            charCount = 1;
        } else {
            charCount++;
        }
    }

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