How to deal with the td tag whose colspan == ''?

a piece of html code:

<td class="advisor" colspan="">

I use beautifulsoup to obtain the information of it.

Here is my code:

soup = BeautifulSoup(html, 'html.parser')
tds = soup.find_all('td')

for td in tds:
    if td["colspan"] == '':
        col = 0
    else:
        col = int(td["colspan"])

However, I get this error:

ValueError: invalid literal for int() with base 10: ''

I know this error means '' cannot be transformed to integer, but why my 'if' does not work? I think this situation should go to

col = 0

rather than

col = int(td["colspan"])