Selling stock

[Stuart McGraw]

I am having a problem following the instructions in the Concepts 
Guide for selling stock
  (http://gnucash.org/docs/v2.4/C/gnucash-guide/invest-sell1.html)
Is says to create a split in which the stock asset is entered twice, 
the second time with "number of shares" and "share price" set to 0.

However, when I try to set either to zero, the zero disappears (the
field becomes blank) as soon as I click in a different column.
When I "Enter" the transaction, the same transaction appears in the 
stock account register twice.  (Before I realized what was going on, 
I deleted the second copy, thinking it was an inadvertent duplicate 
transaction, only to have them both disappear!)

Am I doing something wrong, or is this the expected behavior?
(Using Gnucash-2.4.9 with Postgresql backend)