Proximity-Clustering returns only clusters of size 1

[domangi]

I'm trying to use proximity-clustering, but i get all clusters of size 1, even when increasing the threshold to a large value.

$ curl http://localhost:5984/mydb/_design/gc-utils/_spatial/_list/proximity-clustering/geoms?threshold=5000
{"rows":[
{"points":["307ccb7ec8224fac162a4c2ef658e703"],"size":1},
{"points":["7777350c9e2eaf03d8bc7107445e1128"],"size":1},
{"points":["8a2434e206ce1ac804405ecbdabf8412"],"size":1},
{"points":["307ccb7ec8224fac162a4c2ef6d77256"],"size":1},
...
]}

my documents are structured according to the example on https://github.com/vmx/geocouch-utils:

{
   "_id": "c0c048ad2770bb836a10f164cc0a3fc0",
   "_rev": "1-e2d2130da93ca435965d6d3efca22380",
   "geometry": {
       "type": "Point",
       "coordinates": [
           48.417,
           9.983
       ]
   },
   "etc" : "..."
}

The coordinates of each document are near to each other, since they refer to locations of a single city:

[41.9076, 12.4542]
[41.8818, 12.5173]
[41.9025, 12.4965]
[41.9869, 12.5642]
[41.9025, 12.4965]
[41.9065, 12.4845]
[41.8702, 12.4886]
[41.8905, 12.4678]
[41.8797, 12.4748]
...

I installed geocouch utils through CouchApp command line util, like explained on https://github.com/vmx/geocouch-utils.

Any idea why all clusters have size 1?