Sublime text selects items in the auto-completion panel by the entered characters the so called prefix. The prefix includes words only. As you triggered auto-completion by entering the . the prefix is empty and ST can't know what you want. Just start typing C and you'll get all entries containing this letter.
Auto-completion works on word-base only. Plugins providing context sensitive auto-completions have the chance to inhibit such word completions to cause only valid entries to be added. This is nothing ST does out of the box.
