openssl aes 256 cbc strange bug

This is extremely strange but I have already tried this twice and am completely sure it is openSSL and not me doing the bug. I am encrypting the data 'Hygiene' with the password '*' (sorry can't tell you this) and get the ciphertext '3/mEwtZdIuIV5wwsQAcnAw==' then I changed the last charecter to '-' and when I decrypted it still decrypted normally!. I then tried changing it to different things such as '+' but it didn't work, Is this supposed to be so? Or have I just found another bug in openSSL?