Error in parsing TFormula expression using pre-defined functions in function names

Formula get confused when doing something like

TFormula  f1("f1gaus","gaus+1");

TFormula f2("f2","f1gaus+2");

and it expand the f1gaus name to a gaus function. If a name without numbers is used 

(e.g. fgaus), it works correctly

see also https://root-forum.cern.ch/t/understand-enumeration-of-tf1-parameters/39692/11