Hi Mala, yes it's because one of your objects there is not a closed solid.

You can tell by selecting it and looking at the object type indicator in the upper-right corner of the window. If it's a solid it will say "Solid" there, otherwise if it says "Surface" or "Joined srf" it's not a solid and it means there are some "Naked edges" in the model which are edges that belong to only one face instead of being joined between 2 faces.

Those areas are holes in the "skin" of the object preventing it from being a watertight solid, they need to be sealed off:





You can highlight naked edges to see where they are by selecting an object and pushing the "Details..." button. There will be a report listing the number of joined edges and naked edges in the object and you can click on the naked edges label to select them to see where they are.

You can also set up a keyboard shortcut to show naked edges as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

- Michael