When I enter this code:

de3a := diff(x(t), t) = (x(t))(1-.5*x(t)-.5*y(t)); de3b := diff(y(t), t) = (y(t))(-.25+.5*x(t));
sys3 = {de3a, de3b};

window := x = -1 .. 3, y = -1 .. 3;
DEplot(sys3, [x(t), y(t)], t = 0 .. 14, window);

I get the error message "Error, (in DEtools/DEplot) system must have same number of dependent variables as DE's." What am I doing wrong? I'm just trying to create a plot on the phase plane.