The WriteConcern module has a private method unacknowledged? that determines whether write concern options correspond to an acknowledged write concern. Despite being private to the WC module, this method is called from other places in the driver where it receives user input. In particular, the user input may have string keys rather than symbol keys, and calling unacknowledged? on string key write concern options silently produces wrong results.

Furthermore, the method itself is not correctly implemented. Constructors of both acked and unacked write concern classes convert strings to symbols, but unacknowledged? method does not. Thus when given input of {{

}} the WC#get call will create an acknowledged write concern with w:0 which is wrong.