question on the animated star trek

[TonyStark]

i've been going through the production codes of the episodes of star trek the animated series, and i noticed there is no number 12. the codes go from 1-11, and then from 13-23. does this mean there's some lost episode out there?

 

[C. Lee]

Here they have no production #21 http://epguides.com/StarTrekTheAnimatedSeries/

 

[TonyStark]

Here they have no production #21 http://epguides.com/StarTrekTheAnimatedSeries/

that's bizarre. i'm getting mine from the official star trek site though. oh well, i dunno.