Which SDK version are you using? (The is_busy flag has not been used since SDK 10, so I will assume you are using version 10.)
If you are using the driver in blocking mode (as it looks like you are doing from the code snippet in an earlier comment where you supplied NULL as the event_handler), is_busy should never be set to true. The only exception I can think of is if you initiate another transfer before the first has finished, which is a bit odd since the call is blocking, but it could happen if you call the driver from two different interrupt priorities (for example from the main context and an event handler).
