When you fetch command line arguments, they are always strings: they are "what the user typed on the command line to execute your application" - and you can supply them that way by opening a command prompt and typing the name of your app followed by space-separated parameters:
myApp Parameter1 "parameter 2 with spaces in"
As such, they don't have a type in the sense of "integer" or "decimal".
Use TryParse to check them:
Dim value As Decimal
If Not Decimal.TryParse(Environment.GetCommandLineargs(1), value) Then