Error using faster RCNN object detection

I am getting below error white trying to detect objects using a trained object detector, please help. Getting the error at the line detect(detector, img).

net=alexnet;
detector = trainFasterRCNNObjectDetector(trainingData, net, options)
[bboxes, scores, label] = detect(detector, img);

Error using activations Expected layer to be a scalar.

Error in vision.cnn.FastRCNN>iValidateAndParseLayerID (line 296) validateattributes(layerIdx, {'numeric'},...

Error in vision.cnn.FastRCNN/internalActivations (line 203) layerID = iValidateAndParseLayerID( layerID, internalLayers );

Error in fasterRCNNObjectDetector/detect (line 482) convmap = internalActivations(this.Network, Iroi, [], ...