"COPY_ERROR: Unable to copy" while using drush make with download type "copy" Here is my module structure:

.
|-- mymodule.make
|-- mymodule.info
|-- mymodule.module
|-- mymodule.install
`-- sub1
    |-- sub1.make
    |-- sub1.info
    |-- sub1.module
    `-- sub1.install


I would like that when using drush make the sub-module sub1 is also made (so I would like that sub1.make is called too).

In mymodule.make I declare:

api = "2"
core = "7.x"

projects[drupal][version] = "7.39"

; Modules
projects[sub1][type] = module
projects[sub1][download][type] = copy
projects[sub1][download][url] = "./sub1"


And here is the error that I have:

Found makefile: mymodule.make                                [ok]
COPY_ERROR: Unable to copy sub1 from ./sub1. -- build forced [warning]


However, the documentation tells


  download[type] = copy Copies a project from a local folder.


so I think I'm using the correct [download][type]...

I've also tried with projects[sub1][download][url] = "file://./sub1" and have the same error.
As a workaround, I used the includes[] directive instead.

Here is my working mymodule.make:

api = "2"
core = "7.x"

projects[drupal][version] = "7.39"

; Modules
includes[] = ./sub1/sub1.make