Thanks for your response. I didn't know that the private key was formatted like a Bitcoin address (version byte plus checksum bytes at the end). Is it possible to import a raw private key (i.e. a 256 bit random number) or is mandatory to format the key in that way?
Unless your wallet has some import raw key mode, you'll have to format in the wallet import format:
https://en.bitcoin.it/wiki/Wallet_import_format