Why does the game tell me I am using too much iron?

I am constantly getting the notification that I am using more iron than I have. However, I have some iron and it does not show up as in use:

resources panel showing iron: 7 total, 0 used

How can I find out where my iron is in use? Should that use not show up in this information panel? And how can I see how much iron I need to obtain?


It seems this is a bug in the game. I can't reproduce the circumstances under which it happened.

I saved the game, restarted the game application and loaded the saved game again. Afterwards the message was gone.