# Upside-down trees in forest

I'm trying to typeset a tree which grows upwards using the forest package. The problem I have is that as soon as I leave the first level, branches start growing back down again. Here's the code:

\begin{forest}{grow=north}
[A [B [a][b]] [C]]
\end{forest}


This gives me a tree looking like:

How can I fix this?

\documentclass{standalone}