Value error

[jithin123]

for data in alldata:
    if (data['Familysize']==1):
        data['isalone']=1

What is the error in this simple code?

Error:
ValueError                                Traceback (most recent call last)
<ipython-input-20-853203b87769> in <module>
      1 for data in alldata:
----> 2     if (data['Familysize']==1):
      3         data['isalone']=1

~\Anaconda3\lib\site-packages\pandas\core\generic.py in __nonzero__(self)
   1553             "The truth value of a {0} is ambiguous. "
   1554             "Use a.empty, a.bool(), a.item(), a.any() or a.all().".format(
-> 1555                 self.__class__.__name__
   1556             )
   1557         )

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().