新手请教openbugs unable to generate initial values问题

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 <- 1/exp(theta);

Y~ dnorm(0,Yisigma2);

}

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

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);

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

theta0 ~ dnorm(mu,itau2);

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 <- mu + phi*(theta[i-1]-mu) + psi*(theta[i-2]-mu);

theta ~ dnorm(thetamean,itau2);

}

}