lsode problems

[CarlB]

I have been messing with lsode for a few days and I just can't get it to
function without errors.
*M-file:*
clear all
clc

% Time span
tspan = [0 1]';

% Initial Conditions
IC = [0 0]';

% ODE Solver
[z,t] = lsode(@odem5,tspan,IC)

*Function*
function zdot = odem5(z,t)


%% if statement for step input f
if t < 0 
    f = 0
elseif 0 < t >= .4
    f = 600
else 
    f = 0
endif

zdot = zeros(2,1);
zdot(1) = z(2);
zdot(2) = (-12*z(2) - 900*z(1) + .15*f);
endfunction

*This returns the error:*
 LSODE--  REPEATED CALLS WITH ISTATE = 1 AND TOUT = T (=R1)
      In above message,  R1 =  0.0000000000000D+00
 LSODE--  RUN ABORTED.. APPARENT INFINITE LOOP
f = 0
error: lsode: exception encountered in Fortran subroutine dlsode_
error: called from

any help would be great,

Gus



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