trouble with TermFreqVector in lucene.net

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C#

can anyone tell me how to use the function
TermFreqVector in lucene.net

when i use

IndexReader reader = IndexReader.Open(dir);     
TermFreqVector termFreq = reader.GetTermFreqVector(1, "Content");
String[] term = termFreq.GetTerms();

it gives error as
Object reference not set to an instance of an object.
on the LINE

string[]   term = termFreq.GetTerms();