What is the cannot unpack non-iterable error here?

choice2 = input("Encrypt(e) or Decrypt(d): ")
        cipherlist = []
        keylist= []
        key = input("Input a key: ")
        key = key + 'abcdefghijklmnopqrstuvwxyz'
        removedDups ="".join(OrderedDict.fromkeys(key))
        for val in range(5):
            dupslice = removedDups[(val*5):((val+1)*5)]
            keylist = list(dupslice)
            cipherlist.append(keylist)
                
        #encrypt playfair
        if choice2 == 'e':
            plaintext = input('Input text to be encrypted: ')
            for val in range(len(plaintext)):
                diaplain = plaintext[(val*2):((val+1)*2)]
                plainltr1 = diaplain[0:1]
                plainltr2 = diaplain[1:2]
                (c1, r1) =index_2d(cipherlist, plainltr1)
                (c2,r2) =index_2d(cipherlist, plainltr2)

I am getting a cannot unpack non-iterable NoneType object error on lines: (c1, r1) =index_2d(cipherlist, plainltr1) and (c2,r2) =index_2d(cipherlist, plainltr2). For some reason if I print(cipherlist) before asking for the index it works. Oh I almost forgot, the function I am using for index:

def index_2d(alist, ltr):
    for y, x in enumerate(alist):
        if ltr in x:
            return (y, x.index(ltr))

Thanks so much!! I am stumped as why to this is happening.