calcVal function referring a curve

Hi,

I want to use calcVal to cite one transient signal from test_A to test_B. The format I tried is

calcVal(XXX "test_A")    // XXX is signal name

In the result page, it shows a normal sign as a curve, but when i try to plot it, there is only one empty viva window pop out, and I got error in CIW as

It seems the expression syntax is wrong. 

My virtuoso versio is IC618 64b 500.22

Anyone can help?

BR