I'm trying to use the box select to select multiple objects in Object Mode, then Click "CTRL-J" to join the objects. Sometimes doing this results in the error message: "Active Object is not a selected mesh". It seems to be intermittent. I verified that the selected objects are meshes. It works by selecting objects individually and doing the "CTRL-J", but not always using the box select tool.
What could the problem be? Using Blender 2.75a
EDIT: After looking at this more, I think the problem is that the error message is reversed. I think it should say: "Selected Object is not an active mesh", instead of "Active Object is not a selected mesh".