Error in optim:Function cannot be evaluated at initial parameters - r

Question

So I want to estimate 5 parameters consist of intercept, age, disease, mdu, and alfa from panel data. And I write the pdf, here's the code of multivariate negative binomial for panel data. I use optim() package to estimate the parameters. Please help me to fix what is wrong with my code..
library(pglm)
library(plm)
data("HealthIns")
dat<- pdata.frame(HealthIns,index = c("id","year"))
y<-data.matrix(dat$mdu)
y[is.na(y)]=0
Y<-matrix(data=y,nrow=5908,ncol=5)
dat$ageclass<-ifelse(dat$age >=30,1,0)
x1<-data.matrix(dat$ageclass)
x1[is.na(x1)]=0
X1<-matrix(data=x1,nrow=5908,ncol=5)
dat$gender <-ifelse(dat$sex=="male",1,0)
x2<-data.matrix(dat$gender)
x2[is.na(x2)]=0
X2<-matrix(data=x2,nrow=5908,ncol=5)
x3<-data.matrix(dat$disease)
x3[is.na(x3)]=0
X3<-matrix(data=x3,nrow=5908,ncol=5)
Function for optim package
po.gam=function(para){
#Lambda(i,t)
{for (i in (1:5908)){
for(t in (1:5)){
lambda<-matrix(para[1] + para[2]*X1 + para[3]*X2 +
para[4]*X3,nrow=5908,ncol=5)}}
}
#Sigma N(i,t) terhadap t
num.claims.of.t <-numeric(nrow(Y))
{for (i in seq(nrow(Y))){
num.claims.of.t[i] <-sum(Y[i,])}
}
#Sigma Lambda(i,t) terhadap t
num.lambda.of.t<-numeric(nrow(Y))
{for (i in seq(nrow(Y))){
num.lambda.of.t[i]<-sum(lambda[i,])}
}
#Produc Exponential Dist
prod.exp<-numeric(nrow(Y))
{for (i in seq(nrow(Y))){
prod.exp[i]<-prod(lambda[i,]^Y[i,]/factorial(Y[i,]))}
}
#JOINT PROBABILITY OF TIMESNYA...
joint.pdf.mvnb<-prod.exp*gamma(num.claims.of.t + (1/para[5]))/gamma(1/para[5])*((1/para[5])/(num.lambda.of.t + (1/para[5])))^(1/para[5])*(num.lambda.of.t + (1/para[5]))^(-num.claims.of.t)
#PRODUC NUMBER OF CLAIMS SEMUA INDIVIDU
-log(prod(joint.pdf.mvnb))
}
start.value <- c(beta0=1,beta1=1,beta2=1,beta3=1,alfa=1)
MLE_estimator<-optim(start.value,po.gam,hessian=TRUE)
MLE_estimator
And here is my result
> MLE_estimator<-optim(start.value,po.gam,hessian=TRUE)
Error in optim(start.value, po.gam, hessian = TRUE) :
function cannot be evaluated at initial parameters