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LMDB: Compare function implementation

Hi all,
I am inserting a non-zero length key into a database where I provide a compare function.  But I am seeing that MDB_val->mv_size = 0 in the compare function. Is that possible?

More details (LMDB 0.9.11 Release):

I have a database where I want to delete all elements that have the same prefix.

void eraseElements (KeyPrefix prefix, MDB_txn *txn, MDB_dbi database) 
  MDB_val k;

  k.mv_data = &prefix;
  k.mv_size  = sizeof (KeyPrefix);

  MDB_val v;

  MDB_cursor *cur;
  mdb_cursor_open (txn, database, &cur);
  while (! mdb_cursor_get (cur, &k, &v, MDB_SET_RANGE))
      if (memcmp (&prefix, k.mv_data, sizeof (KeyPrefix)) != 0) break;
      mdb_cursor_del (cur, 0);

        k.mv_data = &prefix;
        k.mv_size  = sizeof (KeyPrefix);
    mdb_cursor_close (cur);

Relevant stack trace
  DB::compare (a, b) and b->mv_size == 0
  mdb_cursor_set(mc, key, data, op=MDB_SET_RANGE, exactp=0x0) at mdb.c:5348
  mdb_cursor_get(mc,key,data,op=MDB_SET_RANGE) at mdb.c:5655