VB6 IsMissing() and optional arguments

In VB6/VBA the IsMissing function is used to test for optional arguments passed to user-defined function procedures as variants. Default values were not required for optional arguments; if no value was passed then IsMissing returned true. It was also possible to specify a default value of a primitive type to pass to the Variant argument.

The important point is that IsMissing only work for optional argurment of Variant Type.
If the optional argument is not of variant type, you should not use the IsMissing() function.
In this case always supply a default value, even if the compiler does not require it.