Wrong negative values on xively

[giardia]

Hello. I'm a newbie. So i just copy from an example script to make a feed in xively.

!/usr/bin/env python

import xively
import time
from datetime import datetime
import HTMLParser
import urllib

key= 'hidden'
feedid = '117061'
urlText = []

class parseText(HTMLParser.HTMLParser):
def handle_data(self,data):
if data != '\n' and data != '\r\n' and data != '\r\n\t':
urlText.append(data)

lParser = parseText()

lParser.feed(urllib.urlopen('http://www.idrografico.roma.it/ASP.NET/tempo_reale_ripetta.aspx').read())
lParser.close()

client = xively.Client(key)
now = datetime.now()
prelev = urlText[35]
lev = prelev.replace(",",".")
if lev > 0:
datastream = xively.Datastream(id="livello", current_value= lev)
client.put('/v2/feeds/'+feedid, data={'datastreams': [datastream]})
print "%s %s" % (now, lev)

The script app.py is parsing water_level data from a website and posting value in xively.
I'm using Heroku platform who run "python app.py" every hour thanks to Heroku Scheduler. (using venv , with git+git://github.com/xively/xively-python.git in the requirements.txt)

The data seem to be sent fine to xively but, every now and then, on a period of 1 week, it shows strange negative values like -999, as you can see from the graph at https://xively.com/feeds/117061/

Why this negative values are posted? They don't seems to come from the website parsing. (i mean, water_level cannot go under zero). How can i solve the issue?

Thanks.