Cannot read property of another product

In a project of mine, I want to build the unit tests with the rpath set, so that they can be run even uninstalled with no need to specify environment variables. With qmake I would use something like

QMAKE_LIBDIR += \
    $${TOP_BUILD_DIR}/lib/Mappero
QMAKE_RPATHDIR = $${QMAKE_LIBDIR}

and, according to this blog post, in QBS I should be able to achieve the same with

Depend { name: "Mappero" }
cpp.rpaths: [ product.Mappero.buildDirectory ]

Unfortunately, though, the expression product.Mappero.buildDirectory evaluates to undefined, and after some debugging it appears that the same applies to all properties I define in that product (oddly enough, though, the product.Mappero.name properly expands).

I'm attaching a minimal test case; you'll see that the warning line will say:

WARNING: Setting rpath to pippoloundefined

I'm using qbs 1.14 from git (cba28b26cdf93a74b52e1ec1635bee7107cb4995).