Why am I seeing "TypeError: string indices must be integers"?

I'm playing with both learning Python and am trying to get GitHub issues into a readable form. Using the advice on How can I convert JSON to CSV?, I came up with this:

import json
import csv

f = open('issues.json')
data = json.load(f)
f.close()

f = open("issues.csv", "wb+")
csv_file = csv.writer(f)

csv_file.writerow(["gravatar_id", "position", "number"])

for item in data:
    csv_file.writerow([item["gravatar_id"], item["position"], item["number"]])

Where "issues.json" is the JSON file containing my GitHub issues. When I try to run that, I get

TypeError: string indices must be integers

What am I missing here? Which are the "string indices"?

Here's a bit of my JSON content:

{"issues": [{"gravatar_id": "44230311a3dcd684b6c5f81bf2ec9f60", "position": 2.0, "number": 263...