请问一下 ，为什么这个计算考试成绩的会出现这个问题

问题遇到的现象和发生背景

# 计算考试成绩

def calculateGrade(request):
if request.method == 'POST':
sid = request.POST.get('sid')
subject1 = request.POST.get('subject')
student = models.Student.objects.get(sid=sid)
paper = models.TestPaper.objects.filter(major=student.major)
grade = models.Record.objects.filter(sid=student.sid)
course = models.Course.objects.filter(course_name=subject1).first()
now = datetime.now()
# 计算考试成绩
questions = models.TestPaper.objects.filter(course__course_name=subject1).
values('pid').values('pid__id','pid__answer','pid__score')

    stu_grade = 0  # 初始化一个成绩
    for p in questions:
        qid = str(p['pid__id'])
        stu_ans = request.POST.get(qid)
        cor_ans = p['pid__answer']
        if stu_ans == cor_ans:
            stu_grade += p['pid__score']
    models.Record.objects.create(sid_id=sid, course_id=course.id, grade=stu_grade,rtime=now)
    context = {
        'student': student,
        'paper': paper,
        'grade': grade
    }
    return render(request, 'index.html', context=context)

我的解答思路和尝试过的方法

我想要达到的结果

stu_grade += p['pid__score']
问题或许应该出现在这里，你print一下这个p['pid__score']，看看是不是返回了None