multi dimensional optim problem

I am attempting to use optim to solve a neural network problem.  I would like to optimize coefficients that are currently stored in a matrix

Y=270 x 1
X= 27- x 14
b1= 10x14
b2= 11x1
V= 10 x 14 set of prior variances.

I have the following function:

posterior.mode1=function(y,X,b_0,b2,V) {

log.like=function(b1) {
  a_g=compute(b1)
  z_g=tanh(a_g);
  z_g=cbind(1,z_g)
  p=softmax(z_g%*%b2);
  a=sum(y*log(p)+(1-y)*log(1-p));
  return(a);
}

compute=function(b1) {
  a_g=NULL;
  for(i in 1:nrow(b1)){
    a_g=cbind(a_g,X%*%b1[i,])
  }
  return(a_g);
}

log.posterior=function(b1) {
  -log.like(b1)+1/2*t(as.vector(b1))%*%diag(as.vector(V))%*%as.vector(b1)
}

a=optim(b_0,log.posterior,method="CG",hessian=TRUE)
return(a);
}

When I run

posterior.mode1(y,X,b1,b2,b1)



I get the following error



Error in 1:nrow(b1) : argument of length 0