--- a/scripts/general/interp3.m
+++ b/scripts/general/interp3.m
@@ -69,14 +69,14 @@
   extrapval = NA;
   nargs = nargin;
 
-  if (nargin < 1)
+  if (nargin < 1 || ! isnumeric (varargin{1}))
     print_usage ();
   endif
 
   if (ischar (varargin{end}))
     method = varargin{end};
     nargs = nargs - 1;
-  elseif (ischar (varargin{end-1}))
+  elseif (nargs > 1 && ischar (varargin{end - 1}))
     if (! isnumeric (varargin{end}) || ! isscalar (varargin{end}))
       error ("interp3: extrapal is expected to be a numeric scalar");
     endif
@@ -146,3 +146,21 @@
 %! [xxi, yyi, zzi] = ndgrid (xi, yi, zi);
 %! vi2 = interpn(x, y, z, v, xxi, yyi, zzi);
 %! assert (vi, vi2);
+
+%!shared z, zout, tol
+%! z = zeros (3, 3, 3);
+%! zout = zeros (5, 5, 5);
+%! z(:,:,1) = [1 3 5; 3 5 7; 5 7 9];
+%! z(:,:,2) = z(:,:,1) + 2;
+%! z(:,:,3) = z(:,:,2) + 2;
+%! for n = 1:5
+%!   zout(:,:,n) = [1 2 3 4 5;
+%!                  2 3 4 5 6; 
+%!                  3 4 5 6 7;
+%!                  4 5 6 7 8;
+%!                  5 6 7 8 9] + (n-1);
+%! end
+%! tol = 10 * eps;
+%!assert (interp3 (z), zout, tol)
+%!assert (interp3 (z, "linear"), zout, tol)
+%!assert (interp3 (z, "spline"), zout, tol)