walletdump generated the following slightly redacted line:
KwntAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 2000-01-01T00:00:00Z label= # addr=1L...
This base58check string results in 34 bytes (checkable using http://gobittest.appspot.com/PrivateKey). The first byte is documented to be 0x80 so the rest is 33 bytes long. Private keys are always 32 bytes.
What does this mean? How to obtain the private key?
Notice that the private key begins with a K. That means it is a key for a compressed public key, and as per the wiki documentation you linked, the key should have 0x80 prepended to it and 0x01 appended to it. That's an extra 2 bytes, not just 1. Please see here for how to convert between the WIF format and the private key, and in future please use bitcoin.stackexchange.com for support/usage questions rather than GitHub
I missed that 0x01 statement in the documentation. Thanks for clarifying. I was under the impression that this was a bug (at least a documentation but) which is why I filed it here.