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Key-size validation issue

WinSCP version: 5.13.3
Windows version: Windows 10 Enterprise, 10.0.16299 Build 16299

I am trying to generate ECDSA keypairs using the SSH Key Generator (/usr/bin/) tool. Using the -b option to generate a 512 bits key like this (PowerShell syntax):

. "C:/Program Files/Git/usr/bin/ssh-[KEYGENERATOR].exe" -t 'ecdsa' -f 'C:\temp\ecdsa-key' -b 512

The following error shows:
Invalid ECDSA key length: valid lengths are 256, 384 or 521 bits

Notice the 521, where I expected a 512.
However, when I change my call to use the 521, a keypair is generated successfully.

. "C:/Program Files/Git/usr/bin/ssh-[KEYGENERATOR].exe" -t 'ecdsa' -f 'C:\temp\ecdsa-key' -b 521

I haven't been able to validate if the keypair works (and is actually 512 bits), but I'd expect it to, as I can't imagine a keypair can actually be generated with 521 bits.