This always happens if you don't have a secondary weapon equipped when you have unlocked the option to do so.
Some of us just want one bar of skills so in order to prevent the swap animation and possibly losing some active effects will not slot a secondary weapon. Or we can't afford a set piece weapon the same as the primary slot yet, or are missing the drop.

I have the same problem at work with the system that captures secondary phone numbers for accounts on file. You can't leave it blank or the previous number used, even if invalid, gets added back in someday.

It seems programmers don't know how, or even try, to allow empty null values.