Internal Server Error が出てしまう．

モザイクアートを作成するプログラムをpythonとflaskで作り、herokuでデプロイしました．
最初の画面は表示されるのですが、モザイクアートを生成を押すとInternal Server Error が出てしまいます．
ログを見ると
TypeError: The view function did not return a valid response. The function either returned None or ended without a return statement.
と出ていました．

Python
1from flask import Flask, render_template, request
2from PIL import Image,ImageFilter
3import subprocess
4import os
5import random
6import math
7import time
8import numpy as np
9import glob
10import cv2
11
12app = Flask(__name__)
13
14# getのときの処理
15@app.route('/', methods=['GET'])
16def get():
17    return render_template('index.html', \
18        title = 'モザイクアートを作ろう', \
19        message = '設定してください')
20
21# postのときの処理
22@app.route('/', methods=['POST'])
23def post():
24    SourceDir = request.form['source']
25    GoalImage = request.form["goal"]
26    OutputImage="output.jpg"
27    SourceImageSize=(60,40)
28
29    TargetZoom=11
30
31    used_count = 3
32
33
34    def __create_tile(org_image,height,width):
35
36        w, h = org_image.size
37        RectList=[]
38        for y in range(0, math.floor(h / height) + 1):
39            for x in range(0, math.floor(w / width) + 1):
40                height2 = y * height
41                width2 = x * width
42                crap_img = org_image.crop((width2, height2, width2 + width, height2 + height))
43                RectList.append((crap_img,width2,height2))
44        return RectList
45
46    def __clac_hist(img):
47
48        hist_list = []
49        color = ['r','g','b']
50        images = np.asarray(img)
51
52        for i in enumerate(color):
53            hist_list.append(cv2.calcHist([images],[i[0]],None,[256],[0,256]))
54        return hist_list
55
56    if __name__=="__main__":
57
58        startTime=time.time()
59
60
61        target=Image.open(GoalImage)
62        target=target.resize(tuple(math.floor(i*TargetZoom) for i in target.size))
63
64
65        tiles = __create_tile(target,SourceImageSize[1],SourceImageSize[0])
66
67        file_paths = glob.glob(SourceDir+"\*")
68        src_hist_dict = {}
69        for file_path in file_paths:
70            file_name = file_path.split("\")[-1]
71
72            try:
73                src_image =  Image.open(file_path).resize(SourceImageSize)
74            except Exception:
75                continue
76            src_hist_dict[file_name] = [src_image,__clac_hist(src_image),0]
77
78
79        Target=Image.new("RGB",target.size,255)
80
81        while(len(tiles) > 0):
82            result = []
83
84            r=random.randrange(len(tiles))
85            tileRect=tiles[r]
86            del tiles[r]
87
88            tile_hist = __clac_hist(tileRect[0])
89
90            tile_hist = np.array(tile_hist)
91            tile_hist = tile_hist.reshape(tile_hist.shape[0] * tile_hist.shape[1], 1)
92
93
94            for file_name,src_hist in src_hist_dict.items():
95                src_hist = np.array(src_hist[1])
96                src_hist = src_hist.reshape(src_hist.shape[0] * src_hist.shape[1], 1)
97
98                d = cv2.compareHist(tile_hist, src_hist, cv2.HISTCMP_INTERSECT)
99                result.append([d,file_name])
100            result.sort(reverse=True)
101
102            Target.paste(src_hist_dict[result[0][1]][0],(tileRect[1],tileRect[2]))
103
104            src_hist_dict[result[0][1]][2] = src_hist_dict[result[0][1]][2] + 1
105
106
107        Target.show()
108        return render_template("index.html",message="モザイクアート生成完了")
109
110
111if __name__=="__main__":
112    app.run()
113