If I carry out any combat on a Location Type structure, I always get the attached error message. This is where I have set a Landstack limit for the Location Type higher than zero.
If I carry out the combat on a hex without a Location Type, then I do not get the error message. Also, if I edit the Landstack limit to set it back to zero, I also do not get the error message.
So it looks like using the Landstack feature is what is causing the bug. I haven't changed any other variables: e.g. I left all the combat modifiers for the Location Type at "1".
