Interplanetory launcher can't send less than 20kg

Even if the minimum mass option allows values between 2 kg and 20 kg, the interplanetory launcher refuses to launch less than 20kg. :-/

I was trying to send 14kg of wort seeds to another planetoïd, but even if I set the minimum mass to 14 kg or 2 kg, nothing happens !

I was forced to add a random resource so that the mass inside the launcher is higher than 20kg. -_-