MySQL AES_ENCRYPTION of an Existing Colum

I got an example of how to convert an existing column to AES encrypted data:

UPDATE SomeTable SET sensitive_column = AES_ENCRYPT(sensitive_column, 'password');

However when I run this for my scenario I get the following error:

Error Code: 1582. Incorrect parameter count in the call to native function 'aes_encrypt'

Any ideas?