Problem with my doubly linked list insert function - c++

Question

I am trying to finish up a assignment for my data structures c++ class. I have to define a double linked list of functions(like insert() size() remove() ) that was provided by my instructor. The instructor also includes the main file which runs tests on my code to see if it works.
I'm receiving his error message:
* Starting dlist tests *
Checking empty list...
FAILED: size of empty list is != 0.
I tried to rewrite the definition of the size() and insert() function and Im not understanding why im getting his error.
my instructors test code:
bool test_empty() {
std::cout << "Checking empty list...\n";
dlist e;
if(!verify(e))
return false;
if(e.size() != 0) {
std::cout << "FAILED: size of empty list is != 0.\n";
return false;
}
if(!e.empty()) {
std::cout << "FAILED: empty list is not .empty().\n";
return false;
}
return true;
}
my code for implementations:
void insert(node *previous, int value){
if(previous == nullptr){
node* n = new node;
n->value = value;
n->prev = previous;
n->next = nullptr;
return;
}
node* n = _head; //made a pointer to start at the head
while( n!= previous )//make n go down the list until it hits previous
{n = n->next;}
node* store_next = n->next; //store the address of the prev pointer of the next node
node* a = new node;//create the node that will be inserted
a->value = value;
n->next = a;// the pointer n points to the new node
a->prev = n; //the prev in the new node points to the previous
a->next = store_next; //the next in the new node points to the next node
store_next->prev = a; //the next node's prev points to the new inserted node
}
int size() const{
node* n = _head;
int size = 0;
while(n != nullptr){
size++;
n = n -> next;
}
return size;
}
Heres my default constructor and double linked list struct that my professor requires I use
class dlist {
public:
dlist() {
}
struct node {
int value;
node* next;
node* prev;
};