Gradle 3 + Java 6: unmappable character fails build

Hi, using gradle 3.1, I still need to compile project with java 6. I fork another process, using alternative executable for compilation. But the thing is that when I used gradle 2.13 on java 6 and didn’t forked compilation, there was an error unmappable character for encoding utf-8
but it was suppressed and build ended up successfully. Now, when I fork java 6 compilation, it throws the same error, but it also fails the build.

So why this behavior occurs? And how can I suppress this error in a forked process?