Index out of range when recursively collapsing or expanding IOutliningNodes?

I have the following 2 funtions to expand all child nodes of a given node, and one to collapse all nodes. Both of these seem to throw an intermitint exception on the line "child.IsCollapsed = false/true"

'Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index'

It seems every time it happens its setting IsCollapsed to false, when it already is equal to false. 

ver 19.1.684

private void ExpandChildNodes(IOutliningNode node)
{
    if(node.Count > 0)
    {
        foreach(IOutliningNode child in node)
        {
            child.IsCollapsed = false;
            ExpandChildNodes(child);
        }
     }
}

private void CollapseAllNodes(IOutliningNode node)
{
    if (node.Count > 0)
    {
        foreach (IOutliningNode child in node)
        {
            child.IsCollapsed = true;
            CollapseAllNodes(child);
        }

    }
}