division error

[mcgrim]

The following code

class interval:
    def __init__(self,left,right):
        self.right=right
        self.left=left
    def __add__(self,other):
        a,b,c,d = self.left, self.right, other.left, other.right
        return interval(a+c,b+d)
    
    def __sub__(self,other):
        a,b,c,d=self.left,self.right,other.left,other.right
        return interval(min(a,b)-max(c,d),max(a,b)-min(c,d))
    def __mul__(self,other):
        a,b,c,d=self.left,self.right,other.left,other.right
        return interval(min(a*c, a*d, b*c, b*d),
                            max(a*c, a*d, b*c, b*d))
    def __div__(self, other):
        a, b, c, d = self.lo, self.up, other.lo, other.up
        # [c,d] cannot contain zero:
        print(min(a/c, a/d, b/c, b/d))
        
        if c*d <= 0:
            raise ValueError ('Interval %s cannot be denominator because it contains zero' % other)
        
        return interval(min(a/c, a/d, b/c, b/d), max(a/c, a/d, b/c, b/d))
    
    def __repr__(self):
        return '[{},{}]'.format(self.left,self.right)

I1 = Interval(1, 4)
I2 = Interval(-2, -1)
print(I1 + I2)       
print(I1-I2)
print(I1*I2)
print(I1/I2)

gives me the following error:

Error:
File "C:/Users/Desktop/python ode/homework2.py", line 52, in __truediv__
    raise ValueError('Please make sure values is of sort int and x !=0')

ValueError: Please make sure values is of sort int and x !=0

and I am not sure what to do to fix it.