Object reference not set to an instance of an object. on Guid.Parse

I have this line of code in my page lode.

string sId = Request["sId"];
//couple of more initializations
 if (!string.IsNullOrEmpty(sId))
            {
                Guid GuidThingId = Guid.Parse(sId); //line 73
//method continues.

In my production logs, for some users I see Null Exception at line 73. Object reference not set to an instance of an object.

Any idea of what's going on?