Hey, please verify that you are encoding these values right. You need to use URL encoding. I just tried it and it works fine here:
{
"result": {
"places": [
{
"id": "5a110d312052166f",
"url": "https://api.twitter.com/1.1/geo/id/5a110d312052166f.json",
"place_type": "city",
"name": "San Francisco",
"full_name": "San Francisco, CA",
/* Truncated */
If the url encoding is not the issue, please let us know, as it might be an issue with the API then.