OpenSceneGraph CullVisitor::apply(Geode&) detected NaN

Question

I am trying to use OSG for displaying some cubes on the screen.

at some runs it works perfectly but sometimes it does not display anything, just prints this in the virtual console:

CullVisitor::apply(Geode&) detected NaN,
    depth=nan, center=(0 0 0),
    matrix={
        -1 0 0 0
        0 0 1 0
        0 1 0 0
        -nan -nan -nan -nan
}

the reason why it sometimes works and other times doesn't is probably that the cubes are positioned randomly, and some positions apparently do not work.

The question is: what does it mean and how do I avoid it?

note: you may be tempted to downvote this question right away, but please note that google only provides miserably useless results and I see no way of solving this problem other than asking for help.