Why segmentation fault using malloc~?

 dict->start=(DNODE *)malloc(sizeof(DNODE)); // error occurs here

 /* DNODE and dict defined as following:

typedef struct _DICTIONARY {
  DNODE *hash[MAX_HASH_SLOT]; // the hash table of slots, each slot points to a DNODE
  DNODE *start;               // start of double link list of DNODES terminated by NULL pointer
  DNODE *end;                 // points to the last DNODE on this list
} __DICTIONARY;

typedef struct _DICTIONARY DICTIONARY;

extern DICTIONARY *dict;


typedef struct _DNODE {
  struct _DNODE  *next;
  struct _DNODE  *prev;
  void    *data;        //  actual data points to URLNODE
  char key[KEY_LENGTH]; //  actual (URL) key 
} __DNODE;

typedef struct _DNODE DNODE;


Initialization:

  dict->start=(DNODE *)malloc(sizeof(DNODE));  //fault~? why?~
  dict->end=(DNODE *)malloc(sizeof(DNODE));
  dict->start->prev=dict->start->next=NULL;
  dict->hash[0]=dict->start;
  dict->start->data=n;
  BZERO(dict->start->key,KEY_LENGTH);