Python scripting

Hello

I am reading an abaqus odb which is solved for 2 time steps 'step1' and 'step2' and i want to read  the output at every last frame of these steps.  further, I have written python script to find change in dispacement. I have written this code which works fine to capture delta displacement

f1=odb.steps['step1'].frames[-1].fieldOutputs['U']
f2=odb.steps['step2'].frames[-1].fieldOutputs['U']

deltaf=f2-f1

nstep = odb.Step(name='dfield', description='user defined results', domain= TIME, timePeriod=0)
nframe = nstep.Frame(incrementNumber=0, frameValue=0.0)
nfield = nframe.FieldOutput(name='U', description='user disp',type=VECTOR)
nfield.addData(field=deltaf)
odb.save()

 Now I want to do know change in vonMises stresses i.e i want to find out delta von-mises stress. I have written similar code but its not working.

f1=odb.steps['step1'].frames[-1].fieldOutputs['MISES']  this also gives error. Please help.