MFXMuxer_Init gives either AccessViolationException or SEHException

So, I am calling MFXMuxer_Init from within a function in a C dll, everything seems to execute fine until it reaches this line:

MFXMuxer_Init(streamParams, (mfxDataIO *)curBitstream, muxer);

At that point C# throws either AccessViolationException or SEHException for function that is calling it.

Any thoughts regarding what could be going on?

I have placed the code proceeding this that sets up streamParams and calls this function here: http://pastebin.com/3WVr64rP

Here is the BitstreamIO class: http://pastebin.com/t6QDjhrB

Thank you!