Object reference not set to an instance while not being null

Object reference not set to an instance while not being null

I'm getting some unexpected behavior in my process. I'm doing the following.
IEnumerable<Thing> things = ...;
IEnumerable<Thing> subset = things.Where(a => a.SomeFlag);
String info = "null: " + (subset == null);
The above works and info tells me that the object isn't null. So I wish to
check the number of the elements in subset by this.
IEnumerable<Thing> things = ...;
IEnumerable<Thing> subset = things.Where(a => a.SomeFlag);
String info = "null: " + (subset == null);
String count = subset.Count();
Now I get an exception giving me the error message:
Object reference not set to an instance of an object.
What do I miss?!