VAST error 303, aka Empty VAST response returned, is an error that can be caused when no ads are returned in a VAST response after one or more wrappers. This also includes empty VAST responses from video fallback. When working with third-party networks, the fill-rate can be less than 100%. If so, this is an expected error. For example, if the third-party expects to fill 60% of the time, you should expect 40% errors. To fix the error, the publisher should ensure that the error rate is in line with the fill-rate of the third-party network. Video fallback may be helpful to avoid losing impressions and the buyer should ensure that bids are not being returned with an empty VAST response, and that empty responses are not being returned by the advertiser or agency hosting the creative.