[Chimera-users] Error when making Covalent bonds

Hi! I am trying to pull up two molecules in chimera and link them by forming a covalent bond between a N and C atom. I am selecting only the N of one molecule and the C of the other. When I go to command line and use the bond sel command I get the following error message. Type error: Cannot form covalent bond joining two molecules. Does anyone know how to get around this. Thanks, David C.