TString corruption

Dear root experts,

I am facing the following problem : in my code I use a TString variable, as an argument of many functions.
For a reason I don’t understand, after several fine calls of the same function (which uses TTree::Draw with a variable, a histogram, and a selection TCut), at some point the variable gets corrupted and I get the error :

Error in TTreeFormula::Compile: Bad numerical expression : “diTauNSV???ass”

Would you have a clue of what happens ?