The TO-shard of a migration depends on the FROM-shard to provide sharding details in the case that this is the first sharding operation to hit the TO-shard. If the TO-shard was previously initialized in any cluster, however (such as if the primary of the TO-shard was stepped-down and stepped-back-up) the TO-shard will not initialize with the cluster info sent by the FROM shard and cleanly fail to migrate (with a cryptic error message).