Strange error when I want to import data into TMVA classification module

When I try to use signal and background data in TMVAClassificatin.C file. I come across a strange error.Shown below the image.I debug and retry the input data a lot .But it still can not work.How can I sovle this probelm .Many thanks.

Are you sure your input TTree’s contain a branch (variable) with name "a" ?
It looks like the variable is not present and this causes this error


I finally find the tiny bug,I use two different variables in signal (a) and backgroud (a2) trees,which leads to such error information .Thanks so much !!