What is wrong with my delete function for Binary Search Tree

I have a BST and I want to add/delete/find/inorder_traversal/... on it. But when I want to delete an item, there is a problem.

initialize tree:

node_t *init() {
    node_t *node = calloc(1, sizeof(node_t));
    node->left = NULL;
    node->right = NULL;
    node->data = NULL;
    return node;
}

add() function:

int add(node_t *node, void *data) {
    if (node->data == NULL) { //first time when there is only root node exists
        node->data = data;
        printf("%d -> %p\n", (int)node->data, node);
        return 0;
    }
    node_t *prev = NULL, *ptr;
    ptr = node;
    char type;
    while (ptr) {
        prev = ptr;
        if (data < ptr->data) {
            ptr = ptr->left;
            type = 'l';
        } else
        if (data > ptr->data) {
            ptr = ptr->right;
            type = 'r';
        }
    }
    if (type == 'l') {
        node_t *node = calloc(1, sizeof(node_t));
        node->data = data;
        node->left = NULL;
        node->right = NULL;
        prev->left = node;
        printf("%d -> %p\n", (int)node->data, node);
        return 1;
    } else
    if (type == 'r') {
        node_t *node = calloc(1, sizeof(node_t));
        node->data = data;
        node->left = NULL;
        node->right = NULL;
        prev->right = node;
        printf("%d -> %p\n", (int)node->data, node);
        return 1;
    }
    return -1;
}

del() function:

int del(node_t *node, void *data) {
    if (data < node->data) {
        del(node->left, data);
    } else
    if (data > node->data) {
        del(node->right, data);
    } else
    if (data == node->data) {
        if (node->left == NULL && node->right == NULL) {  // leaf node
            free(node);
            return 0;
        } else
        if (node->left != NULL) {       //node has only left child
            node->data = node->left->data;
            node->left->data = 0;
            free(node->left);
            return 0;
        } else
        if (node->right != NULL) { //node has only right child
            node->data = node->right->data;
            node->right->data = 0;
            free(node->right);
            return 0;
        } else
        if (node->left != NULL && node->right != NULL) { // node has two children
            node_t *temp = min_value(node->right);
            node->data = temp->data;
            del(node->right, data);
            return 0;
        }
    }
    return -1;
}

main():

int main() {
    node_t *tree;
    tree = init();
    add(tree, (void*)40);
    add(tree, (void*)10);
    add(tree, (void*)30);
    add(tree, (void*)25);
    add(tree, (void*)50);
    add(tree, (void*)11);
    add(tree, (void*)76);

    preorder_traversal(tree);
    printf("\n");
    del(tree, (void*)30);
    preorder_traversal(tree);
    printf("\n");
    return 0;
}

before deleting, I run inorder_traversal(), then delete number 30, and again run inorder_traversal(), but the output is as below:

40 10 11 25 30 50 76
40 10 11 0 25 50 76

Guys I would appreciate if you help me.

Also, min_value() function is as below:

node_t *min_value(node_t *node) {
    while (node->left != NULL) {
        node = node->left;
    }
    return node;
}