I have written many COM and VSTO add-ins for Visio, but got confused when a client recently reported that the following Custom UI Runtime Error was appearing. I wasn’t getting the same error on any of my test laptops, so I chatted with my good friend, and fellow Visio MVP, John Goldsmith ( John Goldsmith’s visLog (typepad.co.uk) ). He gave me an excellent tip….
![](https://bvisual.net/wp-content/uploads/2021/11/Custom-UI-Error.png)