Abaqus/standard: The fluid cavity surface is not completely enclosed.

I use the 2D axisymmetric model to build a surface-based fluid cavity in Abaqus/standard. The model is showed in the attached picture. The fluid cavity surface is the arc and the refenrence point is the center of the arc.

When I complete the calculation case, it came the following warning messege:

WARNING: "The surface assembly_al-1_surf-pore is not completely enclosed. A list of element with free edges on this surface is given as follows:"

but the history output value CVOL (hydrostatic fluid cavity volume) is right, which is equel to the whole sphere.

I want to ask that what does the warning message mean and how to remove it. Thanks very muc!