Tensorflow object detection: how to detect on batch

I am trying to perform detection on a batch using tensorflow detection tutorial, but the following code gives me setting an array element with a sequence. error.

# load multiple images 
np_images = []
for img_path in img_paths:
    img = Image.open(image_path)
    image_np = load_image_into_numpy_array(img)      
    image_np_expanded = np.expand_dims(image_np, axis=0)
    np_images.append(image_np)

#Get input and output tensors
image_tensor = det_graph.get_tensor_by_name('image_tensor:0')  
boxes = det_graph.get_tensor_by_name('detection_boxes:0')     
scores = det_graph.get_tensor_by_name('detection_scores:0')
classes = det_graph.get_tensor_by_name('detection_classes:0')
num_detections = det_graph.get_tensor_by_name('num_detections:0')

# detect on batch of images
detection_results = sess.run(
        [boxes, scores, classes, num_detections],
        feed_dict={image_tensor: np_images})  

How to feed an array of images correctly?