Normalize a histogram using bin content from another one

Hi all,
I am trying to create a new histogram (hist_new) via normalizing the bin content from another histogram (hist_old). As a first step I am trying to access the ROOT file and clone (hist_old) but for some reason I get a null pointer error. May you please help me with this?
Here is part of my python script:

def fillPileupHistogram (lumiInfo, calcOption, hist, minbXsec, Nbins):   
    f = ROOT.TFile.Open(,'r')
    hist_old = f.Get("BX distribution for fill " + str(fill_num))
    hist = hist_old.Clone()
    return hist

_ROOT Version:_6.10/09
Platform: CERN remote host
Compiler: python
___Preformatted text

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