svmclassify - classperf - ground truth must be cell of strings

Hi,

I'm getting an error using classperf which says "Ground truth must be a cell of strings or a numeric array." I'm not sure why I'm getting this as I'm pretty sure the vectors that are being used as parameters for classperf are cell string arrays. My code is:

load Experiment1.mat;

%%Training
clc; close all;

data1_train = N(1:100, :);
data2_train = NE(1:100, :);

group1_train = repmat('N', size(data1_train, 1), 1);
group2_train = repmat('S', size(data2_train, 1), 1);

data_train = [data1_train; data2_train];
group_train = [group1_train; group2_train];

svmstruct = svmtrain(data_train, group_train, 'showplot', true);

%%Testing
data1_test = N(101:200, :);
data2_test = NE(101:200, :);

group1_test = repmat('N', size(data1_test, 1), 1);
group2_test = repmat('S', size(data2_test, 1), 1);

data_test = [data1_test; data2_test];
group_test = [group1_test; group2_test];

cp = classperf(group_test);

classes = svmclassify(svmstruct, data_test, 'showplot', true);
classperf(cp, cellstr(classes), cellstr(group_test));

Could you please let me know what could be going wrong?

Thank you!