Java Introduction to Functional Programming Code Submissions Encountering Error

In the Java Introduction to Functional Programming course, there are multiple tests where one has to download some code, write their own code, and then submit a zip file. On every single one of these, I am getting the error "Oh no! We encountered an error with your last submission. Here's an example:

Local Code Challenge | Using forEach

package com.teamtreehouse.challenges;

import java.util.Arrays;
import java.util.List;

public class Printer {

  public static void main(String[] args) {
    List<String> chipmunks = Arrays.asList("Alvin", "Simon", "Theodore");
    System.out.println("Imperative results:");
    printChipmunksImperatively(chipmunks);
    System.out.println("Declarative results:");
    printChipmunksDeclaratively(chipmunks);
  }

  public static void printChipmunksImperatively(List<String> chipmunks) {
    for (String chipmunk : chipmunks) {
      System.out.printf("%s! %n", chipmunk);
    }
  }

  public static void printChipmunksDeclaratively(List<String> chipmunks) {
    // TODO: Print out each of the chipmunks using the forEach method on Iterable
    chipmunks.forEach(chipmunk -> System.out.println(chipmunk + "!"));

  }


}

My output is: 10:52:40 AM: Executing ':Printer.main()'...

:compileJava UP-TO-DATE :processResources NO-SOURCE :classes UP-TO-DATE :Printer.main() Imperative results: Alvin! Simon! Theodore! Declarative results: Alvin! Simon! Theodore!

BUILD SUCCESSFUL in 0s 2 actionable tasks: 1 executed, 1 up-to-date 10:52:40 AM: Execution finished ':Printer.main()'.

My declarative results exactly match the imperative results. I'm using a forEach just like we did in the lesson. Why am I getting an error on submission? I've been stuck on this for 2 weeks.