Undefined index: calc_fnc error in graph edit form

Steps to reproduce:

1. Create a graph and pick some item for it. Select a function 'All' for that item. Graph type should be 'Normal' at this point.
2. Change graph type, first to 'Stacked', then change it once again to 'Pie'.

Result:
Undefined index: calc_fnc [graphs.php:628 ? CView->render() ? include() in include/views/configuration.graph.edit.php:416]