None is returned by the Floyd Warshall Algorithm

Code Help
1

I’m perusing this article and attempting to put the floyd warshall algorithm into practice. I’m trying to return the matrix containing the antecedents of matrix P and matrix D containing the weights between the shortest routes between points. I think the issue is with my predecessor matrix because it returns none in one instance.

def floydWarshall(self) :
    
    n = len(self._W)
    D = copy.deepcopy(self._W)
    P = copy.deepcopy(self._W)

    for i in range(n) :
        for j in range(n) :
            if i == j or self._W[i][j] == math.inf :
                P[i][j] = None
            else :
                P[i][j] = i

    for k in range(n) :
        for i in range(n) :
            for j in range(n) :
                if D[i][k] + D[k][j] < D[i][j] :
                    D[i][j] = D[i][k] + D[k][j]
                    P[i][j] = P[k][j]
    return D, P