Villager Trading Window Closes Immediately When Opened

For normal villagers that have a workstation, this isn't an issue.

I have a custom villager which is supposed to have its own trades. I don't want it to have a workstation, otherwise it will replace the custom trades with the default workstation trades. But without a workstation, the custom villager won't let the trading window stay on. When I open it, it immediately closes.

Priority Level is normal, there is an easy work-around (Just giving the villager a workstation) but it removes the custom trades.