Question: Error, (in plots/animate) points cannot be converted to floating-point values

Does anyone know what the beneath error stated after the following coding means, can't find
the solution,     
Although looked in mapleprimes site, and thanks for Joe Riel who pointed out the same problem in 
2010,2 June...

But, it seems there is still no vivid solution for such problem, how to follow


k:=20:
m:=4:

p[0] := 1:

tr := 4:

frame:=100:
omega[n]:=sqrt(k/m):

T[n]:=(2 pi/omega[n]):

mpoint := proc (x)

local force, tr: 

tr := x*T[n]: 

force := piecewise(tau

maximize(int(force*sin(omega[n]*(t-tau))/(m*omega[n]), tau = 0 .. t), t = tr+.1 .. dur) :

end proc:

ball := proc (x, y)
pointplot([x, y], color = blue, symbol = solidcircle, symbolsize = 40): 
end proc:

maxplot := plot('mpoint'(t), t = 0.1 .. y, numpoints = 10*y, adaptive = false):
animate(ball, [t, 'mpoint'(t)], t = 0.1 .. y, frames = frame, background = maxplot) ;

Error, (in plots/animate) points cannot be converted to floating-point values
 

why animate doesn't recognise points in a ball procedure, headache :((
Also, can't understand why maximize commmand can exactly find max points from int commmand only if t=tr+0.1..dur, but if t=0..dur it gives nonsense, can someone explain??