psftp put is throwing error <dir>: open for write: failure when filename is not provided in destination location

Question

I am using psftp's put command to upload files. I am not providing the filename in destination location assuming that the file will be copied with the same name. Here's the help page of put command

psftp> help put
put [ -r ] [ -- ] <filename> [ <remote-filename> ]
Uploads a file to the server and stores it there under
the same name, or under a different one if you supply the
argument <remote-filename>.
If -r specified, recursively store a directory.

If I try to use PUT without filename in the destination location, psftp throws the error open for write: failure

psftp> open user@host
Using username "user".
Remote working directory is /
psftp> put <file_name> ./<dir>/
/<dir>: open for write: failure

Whereas if I provide the filename in the destination location, it works

psftp> put <file_name> ./<dir>/<file_name>
local:<file_name> => remote:/<dir>/<file_name>

Why does the PUT command require filename in the destination location?

Answer 1

What the help page means that the whole <remote-filename> argument is optional, not filename part of the argument.

So you can do this

put <file_name>

It uploads the file to the current remote working directory, under the same file name.

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But you cannot do:

put <file_name> ./<dir>/

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You can do this though:

cd ./<dir>
put <file_name>