Pretrained deep net (Vgg-F)

I have downloaded a pretrained model Vgg-F from the site (https://www.vlfeat.org/matconvnet/pretrained/)

When i try to pass my image without training i got the following error

net = load('imagenet-vgg-f.mat') ;
layer ='fc8'
feat = activations(net,img,layer,'OutputAs','columns');

Check for missing argument or incorrect argument data type in call to function 'activations'.

Error in (line 81)

feat = activations(net,img,layer,'OutputAs','columns');

Please how to solve the problem?