inserting specific elements into vector using R

Question

Let's say I have two vectors, one that includes NA values, and another that is the length of the first vector after dropping the NA values. I am looking to insert the NA values from the first vector into the second vector, while keeping the position of the NA values the same.

a<-c(1,2,3,6,5,NA,4,5,NA,45,6,NA)
b<-c(1,2,4,3,6,5,7,8,40)

This can be done by concatenating each component, but this seems extremely tedious, especially since my data are much more complicated than the above example. Something like

b[which(is.na(a))]<-NA

is what I am looking for, but this of course replaces elements instead of inserting elements like I want. I am at a loss for this even though it seems relatively simple.