java -Djcov.propfile=jcov.properties -jar ${JCOV_HOME}/jcov.jar JREInstr -rt ${JCOV_HOME}/jcov_network_saver.jar JDK13
fails with:
SEVERE : Error instrumenting ‘JDK13/jmod_temp/jdk.security.jgss/classes/com/sun/security/sasl/gsskerb/JdkSASL.class’ - skipped
Exception details: Unsupported class file major version 57

This issue caused by asm library that does not support CVF > 56
org.objectweb.asm.ClassReader ::
ClassReader(
      final byte[] classFileBuffer, final int classFileOffset, final boolean checkClassVersion) {
    b = classFileBuffer;
    // Check the class' major_version. This field is after the magic and minor_version fields, which
    // use 4 and 2 bytes respectively.
    if (checkClassVersion && readShort(classFileOffset + 6) > Opcodes.V12) {
      throw new IllegalArgumentException(
          "Unsupported class file major version " + readShort(classFileOffset + 6));
    }
-----
public interface Opcodes {
  ....
  int V11 = 0 << 16 | 55;
  int V12 = 0 << 16 | 56;
  ...
}

The proposed fix is to update asm.