check real decimal

Hi All,
I have folowing line of code:
measurements.Collarsize2 =

Convert.ToDecimal(DdlCollarsize2.SelectedValue.Replace(",", ".").Replace(".00", ""));

But sometimes I get following errormsg:Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.FormatException: Input string was not in a correct format.
W ...