export LC_COLLATE="C"
If you use a non-POSIX locale (e.g., by setting LC_ALL to en_US), then sort may produce output that is sorted differently than you're accustomed to.
In that case, set the LC_ALL environment variable to C. Note that setting only LC_COLLATE has two problems. First, it is ineffective if LC_ALL is also set. Second, it has undefined behavior if LC_CTYPE (or LANG, if LC_CTYPE is unset) is set to an incompatible value. For example, you get undefined behavior if LC_CTYPE is ja_JP.PCK but LC_COLLATE is en_US.UTF-8.