OpenSSL Error While Creating PFX: Expecting: ANY PRIVATE KEY

Recently had to install a certificate on IIS and didn't have a pfx file, so used openssl to generate one from the certificate and the corresponding private key, but got the following error:

$ openssl pkcs12 -export -in domain.crt -inkey privatekey.txt -out domain.pfx
unable to load private key
...:error:0909006C:PEM routines:get_name:no start line:../crypto/pem/pem_lib.c:745:Expecting: ANY PRIVATE KEY

While investigating, noticed that the private key file they sent was in UTF-8 BOM format, and it looks like OpenSSL doesn't like that.

After converting it to plain UTF-8 (removing BOM), everything worked.

I've had a similar problem when using the authors file with Git LFS.

Note that OpenSSL is not part of Windows, so use WSL. It also works in Git Bash.