declaring an optional param as "object" will cause an error:
"invalid use of an object" - seems the param is not  declared ...

is it a bug or a feature ?

OPTION EXPLICIT

Sub TestCase()

	' ok ...
	MsgBox "1 " & TestParam1(ThisComponent).getName()
	MsgBox "2 " & TestParam2(ThisComponent).getName()
	MsgBox "3 " & TestParam3(ThisComponent).getName()

	' will fail ...
	MsgBox "4 " & TestParam4(ThisComponent).getName()
		
End Sub

Function TestParam1(oDoc as Object, Optional oOptional) as Object
	If IsMissing(oOptional) then
		oOptional = oDoc.getSheets().GetByIndex(0)
	End If
	TestParam1 = oOptional
End Function

Function TestParam2(oDoc as Object, Optional oOptional as Variant) as Object
	If IsMissing(oOptional) then
		oOptional = oDoc.getSheets().GetByIndex(0)
	End If
	TestParam2 = oOptional
End Function

Function TestParam3(oDoc as Object, Optional oOptional as Object) as Object
	If IsMissing(oOptional) then
		Dim oOptional
		oOptional = oDoc.getSheets().GetByIndex(0)
	End If
	TestParam3 = oOptional
End Function

Function TestParam4(oDoc as Object, Optional oOptional as Object) as Object
	If IsMissing(oOptional) then
		oOptional = oDoc.getSheets().GetByIndex(0)
	End If
	TestParam4 = oOptional
End Function