Invalid serialized value for allowedthemes in wp_sitemeta table

[tazotodua]

I dont remember exactly, but what i remember is the following:
before installing WP (or after installation) i have manually deleted all WP themes from wp-content/themes folder.
then i installed some themes... i dont know when, but once the value of allowedthemes row in wp-sitemeta become such:

a:4:{s:13:"twentysixteen";b:1; s:5:"meris";b:1; s:8:"my_theme";b:1;}

i mean, there should be a:3 instead of a:4

after that, i am unable to enable/disable any theme from "NETWORK>THEMES" , as even after clicking "NETWORK ENABLE" for any theme and page loaded, the theme was not activated and there was still "NETWORK ENABLE"
(i have even opened quiestion at SE : ​http://wordpress.stackexchange.com/q/237864/33667).

also, after every clicking the "NETWORK ENABLE", i see that a new value is added in DB TABLE:

after i manually changed a:4 to a:3 it was fixed... what happened?