Improper target for subproject with dependency

[Brocan]

Hello. I have a subdirs project:

TEMPLATE = subdirs

SUBDIRS += \
    app \
    lib
app.depends = lib
lib.CONFIG += no_docs_target

prepareRecursiveTarget(docs)
QMAKE_EXTRA_TARGETS += docs

There is QMAKE_EXTRA_TARGETS += docs in app project, but no docs in lib project.
When I try to "make docs", I get an error:
"No rule to make target 'sub-lib-docs', needed by 'sub-app-docs'. Stop."
because the Makefile contain such text:

sub-app-docs: sub-lib-docs
	@if not exist app\ mkdir...
...
docs: sub-app-docs

and nothing else about sub-lib-docs.
I can try to fix it with CONFIG += ordered, but I don't think, that it is a great idea.