请问这个错误是怎么回事呢？我根本没有引用mul这个模块

import math
import numpy as np
from sympy import *
k=Symbol("k")
def fv_normal_ans(pmt,i,n):
    fv_up=pmt/i
    fv_down=(1+i)**n-1
    fv=fv_up*fv_down
    return fv

a=pv_normal_ans(1000,0.1,4)#在这里报错
print(a)

报错详细内容：
TypeError (note: full exception trace is shown but execution is paused at: )
'Mul' object is not callable
File "C:\Users\70707\Documents\python\test\11.普通年金计算(金融工具包).py", line 21, in (Current frame)
a=pv_normal_ans(1000,0.1,4)

不是第21行么，我看错了？