Integral absolute error /Simulink/ to workspace/Error with input data

i am trying to optimize the pi controller gain with the help of IAE. the model is presented in figure but in the code it is showing th error

"Unable to perform assignment because the left and right sides have a different number of elements.

Error in PSO (line 60)

average_objective(t) = average_objective(t) + Swarm.Particles(k).O;"

i have tried to chnage the To workspace block output like, STRUCTURE,TIMESERIES,ARRAY etc.

Please help me out