fixing the "Both Meshes must be a closed mesh" pro

I used bevel on onr of my cubes. Then I use Boolean union to join this cubes together with other BASIC solids (the ones created under Blender mesh function and I did not manually edit the vertices or faces etc). This works fine. Then when I try to use the resultant object in other boolean operations, Blender complains:

Both Meshes must be a closed mesh

and I confirm that the resultant object from the boolean operation above is the culprit (by attempting to union it with a primitive UV sphere directly from the add mesh function.

Now, how can I fix this problem to make the object resultant from that boolean operation become a “closed mesh” again so that I can use it in other boolean operations?

I would appreciate any prompt help because my assignment is due tomorrow.

Dominik

I didn’t have that problem…

enter edit mode on the resulting mesh, select everything, and hit remove doubles in the edit menu [or the specials menu, hit the W key]