How to use aes-encryption in PostgreSQL?

Question

I tried aes-encryption by using following statement:

SELECT encrypt('test', 'key', 'aes');

which worked, but I am not able to decrypt the value. I inserted it in a field of datatype bytea but I'm not sure if that was the right way.

SELECT decrypt(pw, 'key', 'aes') FROM table WHERE ID = 1;

gives me the error

ERROR: function decrypt(bytea, unknown, unknown) does not exist
LINE 1: SELECT decrypt(pw, 'key', 'aes') FROM tabelle WHERE ID = 7; ^
HINT: No function matches the given name and argument types. You might need to add explicit type casts.

Does that really mean that encrypt() is an existing function, but not decrypt()? How else could I retreive aes-encrypted values?