Error moving elements from a List to a Set.

package com.example.model;

import java.util.Set;
import java.util.HashSet;
import java.util.List;
import java.util.ArrayList;

public class Course {
  private String mTitle;
  private Set<String> mTags;

  public Course(String title) {
    mTitle = title;
    mTags = new HashSet<String>();
  }

  public void addTag(String tag) {
    mTags.add(tag);
  }

  public void addTags(List<String> tags) {
     tags.addAll(mTags) ;  //Copy all elements of a List<String> to a HashSet<String>
  }

  public boolean hasTag(String tag) {
    // TODO: Return whether or not the tag has been added
    return false;
  }

  public String getTitle() {
    return mTitle;
  }

}