Chunk split failed with cannot find split point

User reported that this happened on 1.7.5 with 4 shards when trying to manually split the data following the steps outlined in http://www.mongodb.org/display/DOCS/Splitting+Chunks. From the google-groups thread (http://groups.google.com/group/mongodb-user/browse_thread/thread/eb72f25921d064dd#)

> db.runCommand({split: 'db_client_14881.raw_data_adt', find: {_id:'20100601'}})
{ "cause" : { }, "ok" : 0, "errmsg" : "split failed" }
and this error on the mongos log:
want to split chunk, but can't find split point chunk ns:db_client_14881.raw_data_adt
at: shard0002:10.128.0.174:27018 lastmod: 7|1 min:

{ _id: "20100423.091851-17042.2933.hl7" }

max:

{ _id: "20100610.135539-24674.19100.hl7" }

got: <empty>
This is all that is in the mongod log:
request split points lookup for chunk db_client_14881.raw_data_adt { :
"20100423.091851-17042.2933.hl7" } -->> { : "20100610.135539-24674.19100.hl7" }
Finding the split vector for db_client_14881.raw_data_adt over

{ _id: 1.0 }

keyCount:
457579 numSplits: 0 took 132 ms.
query admin.$cmd ntoreturn:1 command: { splitVector: "db_client_14881.raw_data_adt",
keyPattern:

{ _id: 1.0 }

, min:

{ _id: "20100423.091851-17042.2933.hl7" }

, max:

{ _id: "20100610.135539-24674.19100.hl7" }

, force: true } reslen:69 132ms

Using middle instead of find solved the problem. Possible issue with Chunk::pickMedianKey causing the balancer to not find splitPoint