Question: How do I solve a system of differential equations. Specifically a rate equation

I am trying to solve the rate equations for A<=>B<=>C with k1,k2,k3,k4 as the rate constants.  I put in the equations into Maple but can't seem to get it to solve them.  The initial conditions for A(0) = S, B(0)=C(0)=0

Here is what I plugged into the program:

Parameters(S, k1, k2, k3, k4);

sys_ode := d*A(t)/dt = -k1*A(t)+k2*B(t), d*B(t)/dt = k1*A(t)+(-k2-k3)*B(t)+k4*C(t), d*C(t)/dt = k3*B(t)-k4*C(t);

d A(t)
------ = -k1 A(t) + k2 B(t),
dt

d B(t)
------ = k1 A(t) + (-k2 - k3) B(t) + k4 C(t),
dt

d C(t)
------ = k3 B(t) - k4 C(t)
dt

ics := A(0) = S, B(0) = 0, C(0) = 0;
A(0) = S, B(0) = 0, C(0) = 0

dsolve({ics, sys_ode});
Error, (in dsolve) found functions with same name but depending on different arguments in the given DE system: . It is required an indication of the dependent variables