Unable to cap

Just tried to cap the ends of this moulding and get this error:
“Unable to cap one object. The openings did not have closed, planar loops of edges.”
What is the problem/error?

nO_CAP.3dm (1.2 MB)

Hello - if you ShowEdges > Naked edges you’ll see there is a long-wise naked edge - that means there is a gap or overlap at the ends, however small.



Thanks Pascal. I figured it was something like that but couldn’t find it.