大家好，新人写ARSV模型，不过遇到问题不能运行，显示 unable to generate initial value for node <theta[1]> of type UpdaterAMblock.UpdaterHetro, 实在是不知道什么原因了，model syntex is correct， load data 也没问题，大家能帮我找找原因么？万分感谢，模型代码如下

model{

for (i in 1:N) {
    Yisigma2[i] <- 1/exp(theta[i]);
    Y[i]~ dnorm(0,Yisigma2[i]);
}

###########################

phistar ~ dbeta(20,1.5);
phi <- 2*phistar - 1;
psistar ~ dbeta(20,1.5);
psi <- 2*psistar - 1;
mu ~ dnorm(0,0.1);
beta <- exp(mu/2);
itau2 ~ dgamma(2.5,0.025);
tau <- sqrt(1/itau2);

###########################

isigma2<-(1-phi*phi-psi*psi)*itau2;
theta0 ~ dnorm(mu,isigma2);
thetamean[1] <- mu + phi*(theta0-mu);
theta[1] ~ dnorm(thetamean[1],itau2);
thetamean[2] <- mu + phi*(theta0-mu) + psi*(theta[1]-mu);
theta[2] ~ dnorm(thetamean[2],itau2);
for (i in 3:N) {
    thetamean[i] <- mu + phi*(theta[i-1]-mu) + psi*(theta[i-2]-mu);
    theta[i] ~ dnorm(thetamean[i],itau2);
}

}