Function to check for perfect square not working for large number

Question

I came across an algorithm to tell if a given number is perfect square or not in O(logN) time.

Here is the implementation(JAVA) of the idea.

public boolean isPerfectSquare(long x) {
        if (x <= 1)
            return true;
        
        long low = 1;
        long high = x;
        long mid = 0;
        while (low <= high) {
            mid = low + (high - low) / 2l;
            if (mid * mid == x)
                return true;
            else if (mid * mid < x)
                low = mid + 1;
            else
                high = mid - 1;
        }
        
        return false;
    }

This works fine for numbers like 256, 808201 , etc But fails for numbers like 999966000289.

I cannot figure out why?