Question: Problem with DEplot, please help...

I am trying to create a plot for two differential equations but I'm not getting any graphs with the commands below. I'm not sure what's causing the problem and I've tried everything I can think of. Please help if you can. a := 2*ln(2) b := 1/5*ln(2) dose := proc (t) options operator, arrow; sum(2*Heaviside(t-6*n)-2*Heaviside(t-6*n-1/2), n = 0 .. 10) end proc J:=DEplot([diff(x(t), t) = dose(t)-a*x, diff(y(t), t) = a*x-b*y], [x, y], 0 .. 50, {[0, 0, 0]}, stepsize = .5, scene = ([t, x])) K:=DEplot([diff(x(t), t) = dose(t)-a*x, diff(y(t), t) = a*x-b*y], [x, y], 0 .. 50, {[0, 0, 0]}, stepsize = .5, scene = ([t, y])) ([display])({J, K})
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