After org.jboss.resteasy.core.ServerResponseWriter.getDefaultContentType() collects all MessageBodyWriters for which isWriteable() returns true, it checks their @Produces values against the media types in the Accept header. However, if a given MBW doesn't have a @Produces annotation, it is ignored. If no MBWs with matching @Produces are found, getDefaultContentType() goes on to select a media type from the Accept header.

Here's the problem. Suppose the Accept header is

Accept: isnt/writeable, is/writeable;q=0.9

Then "isnt/writeable" will be chosen, for which (presumably) there isn't a MBW, and "is/writeable", for which (presumably) there is a MBW, will be ignored. The result will be 500 exception stating a MBW can't be found for "isnt/writeable".