If an index build on a hashed index is concurrent with any collection writes, the index may be incorrectly marked multikey. As a result, this index can not be used as the shard key on a sharded collection. 

Original title: shardCollection fails with "couldn't find valid index for shard key" despite index existing

Original description:

I'm attempting to shard a number of existing collections. They were all created and populated in a standalone mongod which has now been converted to a sharded cluster. Some of the collections have sharded successfully, others have failed with the above error. A possible cause of this was that the indexes were in the process of being created (with the background: true option) when the shardCollection command was first run, due to the script used not waiting for the background creation to complete.