diff -r cb3577a42a7c radon.m
--- a/radon.m	Mon Dec 03 12:12:09 2007 +0100
+++ b/radon.m	Mon Dec 03 12:35:34 2007 +0100
@@ -28,14 +28,13 @@
 
 function [RT,xp] = radon (I,theta)
 
-  if (nargin == 0 | nargin > 2)
+  if (nargin == 0 | nargin > 2 | !ismatrix(I))
     print_usage ();
   elseif (nargin == 1)
     theta = 0:179;
   endif
 
-  m = size (I,1);
-  n = size (I,2);
+  [m,n] = size (I);
 
   # center of image