High Fanout nets synthesis encounter

Hi All

              I got the high fan out nets using dbGet command into a list. Then I used bufferTreeSynthesis command and got an error.

The error says...  the net a/b/c/d/n_1 has no driver .. and the buffer tree synthesis aborted.

Actually the net it shows is the  input of a macro which i have included in the design.

So what I have to do to get successful buffer tree synthesis. I am struck here.

thanks,