'git submodules update' ignores credential.helper config of the parent repository

I'm checking out a repository in a non-interactive environment and would like to disable interactive credential helpers. According to [1] it can be done by specifying an empty helper in a local config:

  [credential]
    helper =

But the submodule update command ignores the helper specified in the config of the parent repository. To reproduce it, fetch a repository with submodules requiring authentication and run:

  git submodule init;
  git submodule sync;
  git submodule update;

the 'git submodule update' runs a default credential helper. The only way to disable it is specify helper in command-line:

  git -c credential.helper= submodule update

Is it by design?

[1] http://marc.info/?l=git&m=147136396024768&w=2