failure when calling met.p4()

[michael-pitt]

I try to use met to get the transverse mass of the W boson, and I do in the following way:

met = Object(event, "MET")
wMT = (lepton.p4()+met.p4()).Mt()

Then, I hit Unknown branch MET_eta Error.
Question: Can the p4 function of Object to be able to set ret.SetPtEtaPhiM(self.pt, 0, self.phi, 0) in case of missing Eta/Mass variables? Or is there an easy way to get TLorentzVector for MET?
I'm using

met = Object(event, "MET")
met_p4 = ROOT.TLorentzVector(met.pt * cos(met.phi),met.pt * sin(met.phi),0,met.pt)

for now but it can be more convenient to use the p4() functionality of Object