String field xxx contains invalid UTF-8 data when serializing a protocol buffer

[littleningmeng]

How to fix the error below when I call mutator.Mutate(&message, 4096);

[libprotobuf ERROR google/protobuf/wire_format_lite.cc:577] String field 'xxxxx' contains invalid UTF-8 data when serializing a protocol buffer. Use the 'bytes' type if you intend to send raw bytes.

[vitalybuka]

I guess documentation is misleading https://github.com/google/libprotobuf-mutator#usage
Direct use of protobuf_mutator::Mutator needs some additional work.
Optionally is incorrect. Default implementations of "Mutate" is just good enough for unittests, e.g. it does not preserve UTF8 correctness.

Lib is actively used and supported for https://github.com/google/libprotobuf-mutator#integrating-with-libfuzzer

[littleningmeng]

I change my code to the DEFINE_PROTO_FUZZER type.
Also I got the same error.

[vitalybuka]

Is this a proto2 or proto3
Is this on empty corpus? or it already contains some inputs?

[littleningmeng]

It's proto3 and the message is empty.

[hartwork]

proto3 should enforce well-formed UTF-8 according to what commit 3fc43a0 added to the readme 🤔