gnupg gives me the error "Invalid public key algorithm" when I try to use an old key

When I try to decrypt an old file, gpg2 gives me this error:

gpg: public key decryption failed: Invalid public key algorithm
gpg: decryption failed: No secret key

gpg1 gives me this error:

gpg: Ohhhh jeeee: no decrypt() for 17
secmem usage: 1664/2432 bytes in 5/8 blocks of pool 2432/32768
Abort trap: 6

What is GnuPG public key?

GnuPG uses public-key cryptography so that users may communicate securely. In a public-key system, each user has a pair of keys consisting of a private key and a public key. A user's private key is kept secret; it need never be revealed. The public key may be given to anyone with whom the user wants to communicate.

You get this message when you've imported an old secret key but not the corresponding public key. Go back and import the public key as well.