Question: Problem with spline integrating...

Hello! I have the cubic spline S=S(t) defined on 1<=t<=3, and I need to calculate such integral:int((diff(S,t)^2+1)^(1/2),t=1..3);
When I try to calculate this integral, maple8 says:"undefined"...
What is the problem?

p.s. maple code:
x:=[1,2,3];
y:=[1,0,2];
with(CurveFitting);
S:=spline(x,y,t,cubic);
int((diff(S,t)^2+1)^(1/2),t=1..3);

Thankful in advance, Max.
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