RuntimeWarning: invalid value encountered in double

このコードでの私の目的は、カメラを使用して取得した画像内の黒いオブジェクトの距離を測定することですが、以下のエラーで述べたように、このイベントは発生しません。
コードは次のとおりです。

import cv2
import numpy as np

def make_coordinates(image, line_parameters):
    slope, intercept = line_parameters
    y1 = image.shape[0]
    y2 = int(y1*(3/5))
    x1 = int((y1 - intercept)/slope)
    x2 = int((y2 - intercept)/slope)
    return np.array([x1, y1, x2, y2])

def average_slope_intercept(image, lines):
    left_fit = []
    right_fit = []
    if lines is not None:
        for line in lines:
            x1, y1, x2, y2 = line.reshape(4)
            parameters = np.polyfit((x1, x2), (y1, y2), 1)
            slope = parameters[0]
            intercept = parameters[1]
            if slope < 0:
                left_fit.append((slope, intercept))
            else:
                right_fit.append((slope, intercept))
        left_fit_average = np.average(left_fit, axis=0)
        #right_fit_average = np.average(right_fit, axis=0)
        print(left_fit_average, 'left')
        #print(right_fit_average, 'right')
        left_line = make_coordinates(image, left_fit_average)
        #right_line = make_coordinates(image, right_fit_average)
        return np.array([left_line])


def distance_to_camera(knownWidth, focalLength, perWidth):
    return (knownWidth * focalLength) / perWidth
# The first step in finding the distance from the object or mark in the image is to calibrate and calculate the focal length . So , We need to know ：
# Initialize the distance from a known object to the camera
KNOWN_DISTANCE = 24.0
# Initializes the width of a known object
KNOWN_WIDTH = 11.0

def canny(img):
    hsv = cv2.cvtColor(img, cv2.COLOR_RBG2HSV)
    black_lower = np.array([0, 0, 0])
    black_upper = np.array([180, 255, 30])
    black_mask = cv2.inRange(hsv, black_lower, black_upper)
    kernel = 5
    blur = cv2.GaussianBlur(black_mask, (kernel, kernel),0)
    canny = cv2.Canny(blur, 50, 150)
    return canny

def display_lines(img, lines):
    line_image = np.zeros_like(img)
    if lines is not None:
        for line in lines:
            for x1, y1, x2, y2 in line:
                cv2.line(line_image, (x1, y1), (x2, y2), (255, 0, 0), 10)
    return line_image


def region_of_interest(canny):
    height = canny.shape[0]
    width = canny.shape[1]
    mask = np.zeros_like(canny)

    square = np.array([[(635,478),(0,478),(281,200)]
                       ])
    cv2.fillPoly(mask, square, 255)
    masked_image = cv2.bitwise_and(canny, mask)
    return masked_image

cap = cv2.VideoCapture(0)
_,c_image = cap.read()
marker = canny(c_image)
focalLength = (marker [1][0] * KNOWN_DISTANCE) / KNOWN_WIDTH
print('focalLength', focalLength)
while True:
    _, frame = cap.read()
    canny_image = canny(frame)
    cropped_canny = region_of_interest(canny_image)
    lines = cv2.HoughLinesP(cropped_canny, 2, np.pi / 180, 100, np.array([]), minLineLength=40, maxLineGap=5)
    averaged_lines = average_slope_intercept(frame, lines)
    line_image = display_lines(frame, averaged_lines)
    combo_image = cv2.addWeighted(frame, 0.8, line_image, 1, 1)
    CM = distance_to_camera(KNOWN_WIDTH, focalLength, marker[1][0])
    cv2.imshow("result", combo_image)
    cv2.putText(frame, "%.2fft" % CM,
                (frame.shape[1] - 200, frame.shape[0] - 20), cv2.FONT_HERSHEY_SIMPLEX,
                2.0, (0, 255, 0), 3)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()
cv2.destroyAllWindows()

エラーは下の写真のようです。

>>> %Run mesafe.py
focalLength 0.0
[-3.89346104e-01  4.41339935e+02] left
mesafe.py:18: RuntimeWarning: invalid value encountered in double_scalars
  return (knownWidth * focalLength) / perWidth
[-3.90163497e-01  4.41415626e+02] left
[-3.85150497e-01  4.40625233e+02] left
[-3.93702170e-01  4.43145302e+02] left