How are you getting the CultureInfo object?
This is important because number/currency/date formats are valid only if you
have a fully-qualified culture (region & language), for example, "en-US" or
"fr-CA". Neutral cultureinfo instances (ones that only have language, such
as "en", or "fr", and no region) or invariant cultureinfo instances have
absolutely no formatting or parsing information. For example, a culture-info
instance based on "en" can't have date or number formatting rules, because
formatting can be different between the many English-speaking regions of the
world.
-Rob