Asynchronous I/O not ending on exception (C# - NI4882)

I have some weird things happening in the GPIB class when working with the NI4882 library. When an instrument (in this case a Voltage Standard) is not on, an exception is thrown during device.EndReadString() when trying to query the instrument for the first time. Once this happens, every other GPIB call (even ones to different drivers on different addresses) throws an InvalidOperationException saying that an Asynchronous I/O operation is in progress. 

 

From looking up NationalInstruments docs, it seems like 'EndReadString()' should end this operation. My guess is that it is not doing that when an exception is thrown. I used the 'GetCurrentStatus()' method, and it is returning 'None', which the docs say means that an Async I/O operation is in progress.

 

Any idea how to stop this process? (I tried the 'AbortAsynchronousIO()' method, but that doesn't seem to work)

 

Thanks for any help you can provide.