[gmx-users] Umbrella sampling

[spss4 at iacs.res.in]

Hi all
I am doing umbrella sampling taking distance between COM as a reaction
coordinate. when I am trying to calculate PMF using the following command
 gmx wham -it tpr-files.dat -if pullf-files.dat  -o free.xvg -hist
hist.xvg
I got an error 

Program:     gmx wham, version 2016.3
   Source file: src/gromacs/gmxana/gmx_wham.cpp (line 1821)
    
   Fatal error:
   Unknown file type of . Should be tpr, xvg, or pdo.

my tpr-files.dat is 

    umbrella0.tpr
    umbrella1.tpr
    umbrella2.tpr
    umbrella3.tpr
    umbrella4.tpr
    umbrella5.tpr
    umbrella6.tpr
    umbrella7.tpr
    umbrella8.tpr
    umbrella9.tpr
    umbrella10.tpr
    umbrella11.tpr
    umbrella12.tpr

and pullf-files.dat is

     pullf0.xvg
     pullf1.xvg
     pullf2.xvg
     pullf3.xvg
     pullf4.xvg
     pullf5.xvg
     pullf6.xvg
     pullf7.xvg
     pullf8.xvg
     pullf9.xvg
     pullf10.xvg
     pullf11.xvg
     pullf12.xvg
      

And I am using gromacs-2016.3. Can anyone suggest me something?
Thanks
Sunipa