Fort construction with riverine transport doesn't work in New Orleans

[minipol]

My current game vs Athena is almost over. I wanted to try out some things, like constructing a fort,
which I haven't done before.
I have an active general (even several generals) in NO, 4 x 6 lbs gun, and a riverine transport unit (4 elements).
Also 1 sea transport unit (2 elements).
I can't seem to build a fort.
Any idea why? I've put the guns in the stack with the general, outside, in 1 big stack with the transports, and so on.

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[minipol]

I have 4 x 6lbs guns in Saint Louis, and there, same problem. Only when I add a supply wagon to the stack, I'm able to construct a fort.
It's as if the transport ships aren't detected?
If the supply wagons aren't in the stack, I'm also unable to build the fort. But when adding a transport ship to the stack, the units
are boarding the ship, no?

With a supply wagon unit, I'm able to build the fort


When I put a transport in the stack, I'm not.
Somehow the options I have left are build depot but even that one is greyed out.


Leaving the transport outside of the stack doesn't allow me the fort construction.

[GraniteStater]

Nice experiment. I think, mayhaps, the conclusion may be drawn that it is as you have demonstrated.

You need Wagons, not just elements. Just FYI, that was my first thought, having built a few forts in AACW and having discussed it with opponents.

I don't know what the manual or other sources say, but, again, FYI, I would never have tried Transports. Maybe I'm unimaginative. Anyhow, I can think we may conclude that you need Wagon elements, not waterborne ones.