You need to post some code here to get valuable input from others..
We cannot just see a random error and tell where it went wrong without even concerning about where it came from??
See PostRealCode <- click and TellTheDetails <- click
And before you post any code, see UseCodeTags <- click
You realise Hashtable does not allow null values for keys or values, and will probably throw that Exception if you try to use pass a null parameter to any of its methods. You can verify that by reading the documentation for Hashtable and its get method.