problem with g:clang_format#auto_formatexpr

[CallumHoward]

I have the following in my init.vim:

let g:clang_format#command = '/usr/local/Cellar/llvm/3.8.1/bin/clang-format'
let g:clang_format#code_style = "llvm"
let g:clang_format#auto_formatexpr = 1

and I am using:
NVIM v0.1.5-405-g00fc216
Build type: Release

I do not have the vim-operator-user plugin installed.

When using gql or Vgq (in a .cpp file) the inbuilt formatter is used.
:set formatexpr? gives formatexpr=

If I use :setl formatexpr=clang_format#replace(v:lnum,v:lnum+v:count-1) then the gq commands work as expected.
I am not sure how to call clang_format#is_invalid(), but running call clang_format#is_invalid() gives no response.