If an object in your workspace has an error (for example, if you are trying to join two images of different types), then the object name button will turn red to show that this object contains an error and the value display for the object will show the first line of an error message. To see the full message, pop up the object menu (see Figure 4.11), and click on Recalculate item.
The error indicator at the bottom right of the main nip window (see §4.4) turns red if there is an error somewhere in your workspace. Pressing it cycles through all of the objects containing an error.