Unable to specify multiple things using include_directories

In the colord project we have a lib/colord/foo.h and internally .c files are allowed to either the single file (e.g. if the library itself is building) #include "cd-foo.h" or #include <colord/colord.h>. To support this I tried:

in lib/meson.build:

lib_incdir = include_directories('.')

in lib/colord/meson.build:

colord_incdir = []
colord_incdir += include_directories('.')
colord_incdir += lib_incdir

and then in the various places (e.g. src/meson.build) we have to pass the include_directories:

include_directories : [
  colord_incdir,
],

This fails to configure with:

Meson encountered an error in file src/meson.build, line 26, column 0:
Include directory to be added is not an include directory object.

Ideas?