$collStats returns storageStats that are read from the in-memory WT record store, which can be slightly outdated (the count value is what we typically refer to as the "fastCount" value). Currently, the analyzeShardKey background hook will an error thrown by analyzeShardKey if $collStats returns 0 for the count value. However, it doesn't ignore the error if the count is > 0 but the dataSize is 0. The dataSize value is updated just after the count value, so it's possible that we've updated the count value after a recent insert, and not yet the dataSize value, so this error should be acceptable as well.