diff -r afbef2f579c9 scripts/image/private/colorspace_conversion_input_check.m
--- a/scripts/image/private/colorspace_conversion_input_check.m	Tue Jan 16 21:23:51 2018 -0800
+++ b/scripts/image/private/colorspace_conversion_input_check.m	Wed Jan 17 17:30:41 2018 +0000
@@ -47,8 +47,9 @@ function [in_arg, sz, is_im, is_nd] ...
 
     ## Allow for ND images, i.e., multiple images on the 4th dimension.
     nd = ndims (in_arg);
-    if (nd == 3)
+    if (nd < 4)
       is_nd = false;
+      sz(end+1:4) = 1;
     elseif (nd == 4)
       is_nd = true;
       in_arg = permute (in_arg, [1 2 4 3]);
diff -r afbef2f579c9 scripts/image/rgb2gray.m
--- a/scripts/image/rgb2gray.m	Tue Jan 16 21:23:51 2018 -0800
+++ b/scripts/image/rgb2gray.m	Wed Jan 17 17:30:41 2018 +0000
@@ -59,14 +59,8 @@ function I = rgb2gray (rgb)
   ## Matlab always returning class double, is a Matlab bug (see patch #8709)
   I = rgb * xform;
 
-  ## Restore size if necessary
-  if (is_im)
-    if (is_nd)
-      I = reshape (I, [sz(1), sz(2), sz(4)]);
-    else
-      I = reshape (I, sz(1:2));
-    endif
-  endif
+  sz(3) = 1; # grayscale images have 3rd dimension of length 1
+  I = colorspace_conversion_revert (I, sz, is_im, is_nd);
 
 endfunction
 
@@ -82,9 +76,9 @@ endfunction
 ## Test ND input
 %!test
 %! rgb = rand (16, 16, 3, 5);
-%! I = zeros (16, 16, 5);
+%! I = zeros (16, 16, 1, 5);
 %! for i = 1:5
-%!   I(:,:,i) = rgb2gray (rgb(:,:,:,i));
+%!   I(:,:,1,i) = rgb2gray (rgb(:,:,:,i));
 %! endfor
 %! assert (rgb2gray (rgb), I);