Visio API ‘Cell is guarded’ exception

If you’re using the Visio COM API to generate or modify Visio diagrams, you may have come across the exception ‘Cell is guarded’ when trying to set a cell’s FormulaU property (and probably when setting the Formula property). For me it only happened on a machine with Visio 2007 installed, Visio 2003 was fine. The solution was pretty simple, use the FormulaForceU property instead (although potentially this may lead to unexpected behaviour apparently, worked fine for me though).