Help: Is this the bug of VS.net?

There is one line in my program that removes an entry from a Hashtable by its key, like this m_tIndex.Remove(key); It runs well in most time, --that entry becomes null after the operation. However, in a certain spot, after the key is removed, that entry becomes a strange array of hashtables. I have no idea where that array comes from. This problem is repeatable. Is this a bug of VS.net? Is there any way I can avoid it to happen? Thanks