error: [Errno 32] Broken pipe on flask application

Question

I've never had this error before and I don't even know where to start. This is a simple web app running on flask. Everything seems to be running just fine on the application but it keeps giving me this error, even though everything still works.

Here's the full print out of the error:

Exception happened during processing of request from ('127.0.0.1', 61271)
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 295, in _handle_request_noblock
    self.process_request(request, client_address)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 321, in process_request
    self.finish_request(request, client_address)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 334, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 657, in __init__
    self.finish()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 716, in finish
    self.wfile.close()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/socket.py", line 283, in close
    self.flush()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/socket.py", line 307, in flush
    self._sock.sendall(view[write_offset:write_offset+buffer_size])
error: [Errno 32] Broken pipe

Here's my code:

app = Flask(__name__)
Bootstrap(app)
app.config['FLASKS3_BUCKET_NAME'] = 'spokane-discount-properties'
s3 = FlaskS3(app)


@app.route("/", methods=['GET', 'POST'])
def index():
    context = {
    'image': url_for('static', filename= 'property_home.jpeg'),
    'heading': 'We sell property - At a discount!',
    'landing_video': url_for('static', filename='intro.mp4')
    }
    form = forms.OptIn()
    if form.validate_on_submit():
        validate = requests.get(
            "https://api.mailgun.net/v3/address/private/validate",
            auth=auth,
            params={"address": form.email.data})
        if validate.json()['did_you_mean'] is not None:
            flash('Did you mean {}?'.format(validate.json()['did_you_mean']))
        elif validate.json()['is_valid'] == True and validate.json()['is_role_address'] == False and validate.json()['is_disposable_address'] == False:
            r = requests.post(
            "https://api.mailgun.net/v3/lists/[email protected]/members",
            auth=auth,
            data={'subscribed': True,
                  'address': form.email.data})
            if r.status_code == 200:
                requests.post('https://api.mailgun.net/v3/mg.spokanediscountproperties.com/messages',
                    auth=auth,
                    data={"from": '[email protected]',
                        "to": form.email.data,
                        "subject": "Welcome to Spokane Discount Properties",
                        "html": open('templates/indoc.html'),
                        "o:tag": 'indoctrinated'})
                flash('Thanks, we will notify you when we have more properties')
            else:
                flash('You are already subscribed, we will notify you when more properties are available')
        else:
            flash('Holy guacamole! Best check yo self, this is not a valid email.')
    return render_template('index.html', form=form, context=context)