diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index eed11358baf..be04ad733f0 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,11 @@
+2012-11-23  Wolfgang Jenkner  <wjenkner@inode.at>
+
+	* image-mode.el (image-transform-check-size): Use assertions only
+	for images of type imagemagick.
+
+	Otherwise no error, image-transform-fit-to-{width,height} is
+	silently ignored, as before.  Doc fix.
+
 2012-11-23  Chong Yidong  <cyd@gnu.org>
 
 	* faces.el (color-defined-p): Doc fix (Bug#12853).
diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index 4ac62fbb6fc..0e91567a29a 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -746,8 +746,14 @@ close to a multiple of 90, see `image-transform-right-angle-fudge'."
 	    h)))))
 
 (defun image-transform-check-size ()
-  "Check that the image exactly fits the width/height of the window."
-  (unless (numberp image-transform-resize)
+  "Check that the image exactly fits the width/height of the window.
+
+Do this for an image of type `imagemagick' to make sure that the
+elisp code matches the way ImageMagick computes the bounding box
+of a rotated image."
+  (when (and (not (numberp image-transform-resize))
+	     (boundp 'image-type)
+	     (eq image-type 'imagemagick))
     (let ((size (image-display-size (image-get-display-property) t)))
       (cond ((eq image-transform-resize 'fit-width)
 	     (cl-assert (= (car size)