Minimum volume requirement for Shape Tool??

When trying to create shapes with the shape tool, I've found that there seems to be a minimum volume that the shape can be drawn as otherwise it will simply refuse to draw. This makes trying to do outlines of small buildings in my city map incredibly difficult, and I don't really see what the use of it is.

For the record, inkarnate's engine can handle small shapes, because if I draw a shape of 'acceptable' size, then make it smaller, it still registers and can be edited normally. It's only when I try to draw it with the shape tool.

Thanks in advance!