SJ Radar : This upgrade can not be used on your submarine
 

For the past 4 or 5 patrols I've seen the SJ radar available in the upgrades when returning to base. However, when I try to drag it to my sub, I get the message, "this upgrade can not be used on your submarine". I have since upgraded to the Gato class sub and the same thing happens. I've upgraded now to the Balao class and still I get the same message and can't get the SJ radar. This means I still have no radar and I think it must be near 1944.
Is this a normal bug? Do I have to resort to mods to get the radar I should have by now?

Enough renown for it?

The radar isn't already installed by default is it?

That's what happened to me. I went long enough without using renown for the SD radar (because it's broke anyway so why bother) that they added it to my boat for me. I eventually tried to add it myself, but because I already had it and didn't realize it I was told I couldn't use the upgrade.