Generic C# method taking where the enum value as a parameter

Possible Duplicate:
Create Generic method constraining T to an Enum

Given a generic method that only operates on enum values

static void  method(T enum) where T ?????
{
     // do something with enum...
}

How do I constrain T such that only enum values are accepted? I've tried using struct however this disallows the use calling my method with a nullable enum type.