R column assignment fails for lists

Dear R developers and users,

Consider the object :

a <- data.frame(a=c(1,2), b=c(2,3), c=c("a,b","c,d"), stringsAsFactors = F)
a$c <- strsplit(a$c, ",")

Re-assignment works fine for columns 1 and 2, but fails for column 3. If a
is a valid object, the assignment should work.

a[,1] <- a[,1]
a[,2] <- a[,2]
a[,3] <- a[,3]

Warning message:
In `[<-.data.frame`(`*tmp*`, , 3, value = list(c("a", "b"), c("c",  :
  provided 2 variables to replace 1 variables