null returned when calling field.getMode() #3509

Hello,
when calling field.getMode() in the com.google.cloud.bigquery.Field, I got null returned when I creating field by

public static Field of(String name, LegacySQLTypeName type, Field... subFields) {
    return newBuilder(name, type, subFields).build();
  }

which only set the name and type, do you think this should set the default mode of nullable when not specified a mode