Why isn't my TStringList sorted?

I have a custom sorted TStringList

...

Items.CustomSort(@CompareWords);

      

        
        
        
      

    

... with this comparison function:

function CompareWords(List: TStringList; Index1, Index2: Integer): Integer;
begin
  Result := StrIComp(PWideChar(List[Index1]), PWideChar(List[Index2]));
end;

      

        
        
        
      

    

But after noticing some problems with my code that expects the list to be sorted in order StrIComp

, I created this little check ...

for i := 1 to Items.Count - 1 do
begin
  Assert(StrIComp(PWideChar(Items[i-1]), PWideChar(Items[i])) <= 0);
end;

      

        
        
        
      

    

... and it doesn't work.

Why isn't the list sorted correctly?