Arrays in custom web part and SharePoint Designer

I came across an issue when trying to add a custom web part to a page layout in SPD.
Each time I added it, the web part would error “Cannot create an object of type ‘Systems.Collections.ArrayList’ from its string representation ‘Collection’ for the ‘MyValue’ property”.
I narrowed it down to an ArrayList I had declared as public within the webpart. Changing this from public solved the issue for me.