How to replace empty column as "ND" using ASP.NET C#?

Question

I want to set value for particular empty column in data set data table as "ND" in data set.

How to do it?

 

 What I have tried:

 

if (ds.Tables[s].Rows[i][Columncountvalue + 19].ToString().Trim() == "")
 {  
ds.Tables[s].Rows[i][Columncountvalue + 19].ToString().Trim().Replace("", "ND");
 }

 the above code is i tried to fill the empty column as "ND" in dataset data table but below error is occurs

string cannot be of zero length. parameter name oldvalue