I was facing the same problem because some of the images are grey scale images in my data set, so i solve my problem by doing this
from PIL import Image
img = Image.open('my_image.jpg').convert('RGB')
# a line from my program
positive_images_array = np.array([np.array(Image.open(img).convert('RGB').resize((150, 150), Image.ANTIALIAS)) for img in images_in_yes_directory])