Python Vigenere密码加密方法加密不正确

def vigenere_encrypt(plaintext, key):
    ciphertext = ""
    key_index = 0
    for char in plaintext:
        if char.isalpha():
            key_char = key[key_index % len(key)]
            key_index += 1
            key_shift = ord(key_char.upper()) - ord('A')
            if char.isupper():
                encrypted_char = chr((ord(char) - ord('A') + key_shift) % 26 + ord('A'))
            else:
                encrypted_char = chr((ord(char) - ord('a') + key_shift) % 26 + ord('a'))
            ciphertext += encrypted_char
        else:
            ciphertext += char
    return ciphertext