The string was not recognized as a valid DateTime

I am getting an error :The string was not recognized as a valid DateTime. There is a unknown word starting at index 0.

for this line of code:

fee.EndDate = Convert.ToDateTime((SqlDateTime.Null);
 
any suggestions? What am i missing? I tried fee.EndDate = Convert.ToDateTime((SqlDateTime.Null).ToString()); but that also throws same error
 
Would appreciate any help!!!
 Thanks!

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