There was a bug in the OSL library that caused this. It is fixed in our latest update (V-Ray 3.20.03) which should be out sometime next week. If you need it sooner than that, email our support guys at support@chaosgroup.com and ask for access to the stable builds.
Did a test with the new V-Ray update. The error is gone, but the material is transparent. I tried with both bitmap and vrayHDRI. Am I doing something wrong, or is it still bugged ?