nodejs jsonwebtoken with PEM file: error "PEM_read_bio:no start line"

I know there are a bunch of similar questions here but after hours of reading I cannot find a solution to mine. So:

I am trying to use jwk-to-pem for signing a JSON web token using jsonwebtoken. I have previously achieved this but now the same code does not work. After calling jwt.sign with the generated pem I get error:0906D06C:PEM routines:PEM_read_bio:no start line. I know that common reasons would include wrong linebreaks like \r or generally wrong format. I actually even dug through the core nodejs code and found the line throwing this error.

Countless adjustments and attempts brought no solution. Specifically:

• I made sure that all linebreaks are \n
• I tried different strings for the first and last line, e.g. -----BEGIN PUBLIC KEY----- or -----BEGIN RSA PUBLIC KEY-----
• I made sure that the last line contains a \n as last character
• I tried to write the pem string to disk and re-reading it, using node's fs.
• I compared the pem file to several other files with several editors e.g. using vi -b or regex.
• I tried handing the raw file buffer with the pem content to jwt.sign as well as pem.toString('utf8') as well as every other string encouding supported by node
• I tried to generate a random pem file using linux tools and give that to jwt.sign. Specifically: ssh-keygen -t rsa -b 2048 -f jwtRS256.key
openssl rsa -in jwtRS256.key -pubout -outform PEM -out jwtRS256.pem

What am I missing here? What possible causes I did not think of can this have?

I found a very simple explanation: I was by mistake using the public instead of the private key. The error thrown by openssl/crypto is very misleading.

Using a private pem/key the signing works as expected.