C# Generics - Single Function to pass in a type double and string and have it return a null

I have 2 functions below where I've removed the meat of the work in this equation for simplicity.  The 'meat' is the same for both functions.  I want a single function that I can use for both nullable and non-nullable types.  If thats not possible then at least 1 function that will contain the actual logic and one function just calls another but for some reason I can't get it going.  The answer most likely will be in the T declaration or the constraints.  Just can't get it to do exactly what I want.

The FindEntryNull function is used where the objects can be null (string for example) where the FindEntry is used where the type can't be null (double for example).

Any help is appreciated.

public Nullable<T> FindEntry<T>(string key, object OneBasedIndex, Nullable<DateTime> asOfDate) where T : struct, IConvertible
        {
                     string item = FindEntry<string>(key);
            if (null == item)
                return null;
            return (T)Convert.ChangeType(item, typeof(T)); 
        }

        public T FindEntryNull<T>(string key, object OneBasedIndex, Nullable<DateTime> asOfDate) where T : class, IConvertible
        {
             string item = FindEntry<string>(key);
            if (null == item)
                return null;
            return (T)Convert.ChangeType(item, typeof(T)); 
        }

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