LINQ not evaluating check for null object

Question

I'm wondering why my linq statement doesn't properly evaluate the null check on the Agency object which is a property of the Users model.

var invalidUsers = this.DbContext.Users.Where(p => p.Agency != null).ToList();    
var invalidUsersList = invalidUsers.Where(p => p.Agency != null).ToList();

When I run the code above, the first line returns a list of all Users, regardless of the Agency object being null or not. However, the second line executes and properly filters the list and returns a list that is correctly returning Users where Agency is not null.