inconsistent map() behavior

[mcmxl22]

I am trying to use map() and range() to make a list of numbers that return as strings. I want to use it to make sure users don't enter numbers when they are supposed to enter letters. I wrote an if/else to test it but sometimes I'll enter
a number and it prints the error but sometimes it prints the number. Is it because it's in a While Loop? I have tried changing line 6 to if enter in str(number_list): but it doesn't make much difference.

number_list = map(str, range(0, 10))

while True:
    enter = input("Enter a letter. ")

    if enter in number_list:
        print("Error!")
    else:
        print(enter)

Output:
Enter a letter. 0
Error!
Enter a letter. 9
9
Enter a letter. 8
Error!
Enter a letter. 3
3