In https://github.com/google/libprotobuf-mutator/blob/ffd86a32874e5c08a143019aad1aaf0907294c9f/src/mutator.cc#L382, the Boolean result of ParsePartialFromString is not handled (by returning null). This seems like an oversight. But OTOH, it just allowed us to catch a bug in the proto parser library. So maybe it's on purpose? Filing this in any case, so that the libprotobuf-mutator maintainers can decide.