Meaning: The slab or opening boundary is invalid because one of the input boundaries is in error. Recovery: For a slab: Open the Structure task. Click Select . Set the Locate Filter to Slabs. Select the slab in error. Select Define Boundaries on the ribbon. Redefine the boundaries of the slab. Click Finish. For an opening: Open the Structure task. Click Select . Set the Locate Filter to Openings. Select the opening. Select Boundary on the ribbon. Redefine the boundaries of the opening. Click Finish.