Why does my code for covers_all work in the workspace but not when I try to submit it for a challenge?

COURSES = {
    "Python Basics": {"Python", "functions", "variables",
                      "booleans", "integers", "floats",
                      "arrays", "strings", "exceptions",
                      "conditions", "input", "loops"},
    "Java Basics": {"Java", "strings", "variables",
                    "input", "exceptions", "integers",
                    "booleans", "loops"},
    "PHP Basics": {"PHP", "variables", "conditions",
                   "integers", "floats", "strings",
                   "booleans", "HTML"},
    "Ruby Basics": {"Ruby", "strings", "floats",
                    "integers", "conditions",
                    "functions", "input"}
}

def covers(setss):
    lists = []
    for course, value in COURSES.items():
        if value & setss:
            lists.append(key)
    return lists

def covers_all(topic):
  output = []
  for key, values in COURSES.items():
    if topic.issubset(values):
        output.append(key)
  return output