Can't loop k-clustering gap stat function

Hi folks,

I want to loop the following function 20 times to determine the mode of the "OptimalK" output from the evalclusters function. But the error "Error using classreg.learning.internal.DisallowVectorOps/subsasgn (line 28) You cannot assign to an object of class clustering.evaluation.GapEvaluation using () indexing."

If anyone could help with this, and also show how to store the outputted "OptimalK" values that would be great. Thanks for any help!

for r = 1:20
x = DATASET(:,11:14);
eva(r) = evalclusters(x,'kmeans','gap','Klist',[1:20])
end