Unrecognized variable name with selectLabels

Hi everyone,

I'm trying to create a thermal images face detection. So I used the Image Labeler App for creating my ground truth data.

After testing this code on my gTruth object:

 imageDir = fullfile('Dataset', 'Balage', 'Expression', 'exp1');
 addpath(imageDir);

 load('gTruth.mat','gTruth');

 faceGroundTruth = selectLabels(gTruth, 'Face');

I have this error :

Error using groundTruth/selectLabels (line 279) Unrecognized variable name 'Face'.

Error in classifier_test (line 8) faceGroundTruth = selectLabels(gTruth, 'Face');

I don't understand why, because after looking to gTruth.LabelDefinitions, 'Face' is defined...

Thank you in advance for you help !