Questions for building a HashMap in C

Question

I've encountered problems when I'm trying to build a hashMap using C. My goal is to build a hashMap with an array of linkedlist appended.

I'm now working on the insert element function, and here is the strange part. I hope to insert a newNode at the beginning. When I use a temp node head to store the "head" of this linkedlist and try to insert, it fails with no value insert. But if I change the head directly to "_hashmap->arrayOfLists[bucket]", then it works.. I do not know why and I think having a temp node should work.. Anyone can help with this? Thanks!

typedef struct pair{ char* key; char* value; }pair_t;

typedef struct node{ struct node* next; pair_t* kv;}node_t;

[Original problematic code]

void hashmap_insert (hashmap_t* _hashmap, char* key, char* value){

 pair_t* newPair = (pair_t*)malloc(sizeof(pair_t));
 newPair->key = (char*)malloc(strlen(key) * sizeof(char)+1);
 newPair->value = (char*)malloc(strlen(value) * sizeof(char)+1);

 strcpy(newPair->key, key); //copy the input key into the malloc space.
 strcpy(newPair->value, value); //copy the input value into the malloc space.

 //create a newNode that is to be inserted.
 node_t* newNode = (node_t*)malloc(sizeof(node_t));
 newNode->kv = newPair;
 newNode->next = NULL;


 unsigned int bucket = _hashmap->hashFunction(key, _hashmap->buckets);

 node_t* head = _hashmap->arrayOfLists[bucket];

 // if the bucket is empty
  if(head == NULL) {
      head = newNode;
  } else {//insert the newNode at the front.
     newNode->next = head;
      head = newNode;
  }

}

【The code that works]

 if(_hashmap->arrayOfLists[bucket] == NULL) {
     _hashmap->arrayOfLists[bucket] = newNode;
 } else {
     newNode->next = _hashmap->arrayOfLists[bucket];
     _hashmap->arrayOfLists[bucket] = newNode;
 }