behavior of runge-kutta function rk in single step operation inside loop

When trying to use the runge-kutta function rk
in single step mode, inside a loop, I find
peculiar behavior after a few steps. I am solving the
decay equation dn/dt = -n, with n(0) = 1, with the
analytic soln exp(-t) as a test case to get my feet wet.

(%i1) [dt, t, n] : [0.1, 0, 1];
(%o1) [0.1, 0, 1]

/* lots of debug printouts here...  */

(%i2) for j thru 5 do
       (print (j),
        soln2 : rk (-'n, 'n, n, ['t, t, t+dt, dt]),
        print (soln2),
        soln1 : second (soln2),
        print (soln1),
        t : soln1[1],
        n : soln1[2],
        display (t, n))$


1 
[[0,1],  [0.1, 0.904837]] 
[0.1, 0.904837] 
t = 0.1

n = 0.904837

2 
[[0.1,0.904837], [0.2, 0.818731]] 
[0.2, 0.818731] 
t = 0.2

n = 0.818731

3 
[[0.2,0.818731], [0.3, 0.740818]] 
[0.3, 0.740818] 
t = 0.3

n = 0.740818

4 
[[0.3, 0.740818]] 
second: no such element in [[0.3,0.740818]]
 -- an error. To debug this try: debugmode(true);
 -------------------------------------
so do step 4 by hand:
 
(%i3) [t, n] : [0.3, 0.740818];
(%o3) [0.3, 0.740818]

(%i4) rk (-'n, 'n, n, ['t, t, t+dt, dt]);
(%o4) [[0.3, 0.740818],[0.4, 0.67032]]
-----------------------------------------------
so interactively, rk has no problem, but inside
the loop I have trouble at j = 4.


(%i6) display2d:true$
(%i7) build_info();
(%o7) 
Maxima version: "5.28.0-2"
Maxima build date: "2012-08-27 23:16:48"
Host type: "i686-pc-mingw32"
Lisp implementation type: "GNU Common Lisp (GCL)"
Lisp implementation version: "GCL 2.6.8"

Ted Woollett