I believe it may be a localization issue. In VB.NET the decimal separator is a period and a group separators is a , however in some locales this is not the case and they are different.
The following discussion details some potential ways to handle it.
detect-culture-of-number-in-vb-net-i-e-period-or-comma-for-decimal-point-thou
Seeing your code will help pinpoint the best way to handle it.