Problem with a function using rgb2ycbcr

I am trying to do a function using rgb2ycbcr. It converts the image, then shows it up and saves it, but when I call the function, this error appears:

"Subscript indices must either be real positive integers or logicals."

Here's my code:

function x = rgb2yuv( x )
    x = rgb2ycbcr(x)

      imshow(x)
      imwrite(x, 'C:\Users\aaaa\Documents\PDI\Atividade1\imagens\x_rgb2yuv.jpg', 'jpg')
  end

What am I doing wrong?