I have a list named `z` :

```
z<-list( list(a=1, b=2),  list(a=2, b=3), list(a=NULL, b=4))

```

I want this to be converted to a data.frame with the corresponding `a` entry in the `data.frame` assigned as `NULL`. 
Doing this, 

```
do.call( rbind, lapply( z, data.frame, stringsAsFactors=TRUE ) )

```

as expected, gives this error:

```
 Error in data.frame(a = NULL, b = 4, check.names = FALSE, stringsAsFactors = TRUE) : 
 arguments imply differing number of rows: 0, 1

```

What is the work around?