>>That sounds like a gotcha in the VB implicit conversion rules with Option Strict OFF. If you have two overloads (one accepting a string and the other an integer) and attempt to call that function with a decimal value as the parameter then the compiler doesn't know whether it should implicitly convert to a string or an integer (ie which overload to use). You should cast to the type you wish to use prior to the call.
>>(In C# the same construct would give the usual 'invalid argument' error).
>>But I'm not sure why you saw this only at runtime - the compiler should catch it?
>
>That is exactly that. Since I added an overload for a double last night, it started to work.

Because Decimal will implicity widen to Double. You won't lose information but might lose precision. If I worked with VB.NET the first thing I'd do would be to set OPTION STRICT ON. It still allows all the implicit widening conversions but will catch a lot more potential problems at compile time