While trying to find out why the OP's script in this topic didn't work, I thought I discovered that the default value for an upvar is the name of it on the outside, but instead, I also found this bug.
test has an empty definition (I just confirmed that was what causes the bug*).
This script says that a does not exist.
*which makes this not important to fix because with an upvar and without a definition, it practically is the script variables block but with only one variable.