The Precondition Nobody Reads

The Competitive Programmer's Handbook presents this two-pointer scan for finding a contiguous subarray with sum exactly x, and states its input as an array of n positive integers.

def has_subarray_sum(a, x):
    left = total = 0
    for right in range(len(a)):
        total += a[right]
        while total > x:
            total -= a[left]
            left += 1
        if total == x:
            return True
    return False

A teammate deletes the word positive from the docstring. Which input below makes the function return False even though a qualifying subarray exists?