quad函数积分维度不对。

本人用quad函数积分出现下面错误

The integrand function must return an output vector of the same length as the input vector.

程序

syms x y theta
x=cos(theta);
y=sin(theta);
l=quad(@(theta)sqrt((diff(x))^2+(diff(y))^2),0,pi)

算圆的周长.