How to obtain the total mass and COG of more than one subassemblies?

Hello, I would like to obtain the combined mass and COG of more than one subassemblies. Could you please let me know how to do it? I opened a assembly file which contains many subassmeblies. I clicked on one to select it. Then, I shift-clicked another subassembly. Next, I chose Tools->Mass Properties. SW gave me a

"Unable to compute mass properties" error.

SolidworksGeneral