diff --git a/gtk-osx-setup.sh b/gtk-osx-setup.sh
index 82dbd551..41936bb8 100755
--- a/gtk-osx-setup.sh
+++ b/gtk-osx-setup.sh
@@ -100,6 +100,7 @@ export PYTHON_CONFIGURE_OPTS="--enable-shared --with-system-ffi"
 #This really means pyenv's *python* version. It's poorly named but
 #it's defined by pyenv so it can't be changed.
 export PYENV_VERSION=$PYTHON_VERSION
+export PYTHONSTRICTEXTENSIONBUILD=1
 $PYENV install -v $PYENV_VERSION
 PIP="$PYENV_ROOT/shims/pip3"