Why do I get runtime error (NZEC) in EDIST in spoj in Python

Question

I am new to Python and also this problem (https://www.spoj.com/problems/EDIST/) is my first problem in spoj in Python. I am not sure why I am getting runtime error. Is this a format problem? I submitted with Python (cpython 2.7.16)

def edit_distance(s, t):
    sz_s = len(s)
    sz_t = len(t)

    dp = [[0] * (sz_t + 2) for i in range(sz_s + 2)]

    for i in range(1, sz_s + 1):
        dp[i][0] = i
    
    for j in range(1, sz_t + 1):
        dp[0][j] = j

    for i in range(1, sz_s + 1):
        for j in range(1, sz_t + 1):
            dp[i][j] = min(
                            dp[i - 1][j] + 1,
                            dp[i][j - 1] + 1,
                            dp[i - 1][j - 1] + (1 if s[i - 1] != t[j - 1] else 0)
                        )
    
    return dp[sz_s][sz_t]


def main():
    T = int(input())

    for i in range(T):
        str1 = input()
        str2 = input()

        print(int(edit_distance(str1, str2)))

main()