Child items do not appear in QStandardItemModel

[MTK358]

I have a QTreeView set to use a QStandardItemModel, and I add some items to it like this:

QStandardItem *i1 = new QStandardItem("test item");
    QStandardItem *i2 = new QStandardItem("sub item");
    i1->appendRow(i2);
    m_tabModel->appendRow(i1);

QStandardItem *i1 = new QStandardItem("test item");
    QStandardItem *i2 = new QStandardItem("sub item");
    i1->appendRow(i2);
    m_tabModel->appendRow(i1);

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"test item" appears, but "sub item" does not. What's wrong?