undefined method 'map' for nil:NilClass

I'm getting this error while trying to do a ChangeComment:

```
Error detected while processing function codereview#NewChangeComment:
line    1:
NoMethodError: undefined method `map' for nil:NilClass
Press ENTER or type command to continue
```

my vim version:

```
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Jun 20 2014 03:13:00)
Included patches: 1-335
```
