Why can't I build this bridge?

[Lilyacre]

I'm trying to build a diagonal bridge across a river that is 3 tiles thick.

When I try to place the bridge on a straight piece of river that's 3 tiles wide, there are no issues, but for some reason it won't do it diagonally. Has anyone got a diagonal bridge on the 3 tile rivers or can it simply not be done? I'm gutted if not because my whole town has smaller 3-tile rivers and I think the shorter bridges look really nice and the diagonal ones are so much nicer than the straight ones.

 

[Moritz]

I could be wrong but your diagonal river does not look 3 wide to me. It looks like 2

 

[Burumun]

It's three wide horizontally, but only two wide diagonally, from what I can tell.