The Missing Empty Prefix

Counting contiguous subarrays whose sum equals k, using prefix sums in a hash map. It returns 3 for a = [2, 3, 1, 4, -4, 5], k = 5, but the true count is 4.

def count_subarrays(a, k):
    counts = {}
    total = 0
    result = 0
    for x in a:
        total += x
        result += counts.get(total - k, 0)
        counts[total] = counts.get(total, 0) + 1
    return result

Which subarray is missed, and what is the fix?