Visual Basic for Applications Reference
The required identifier argument is a variable name.
IsObject is useful only in determining whether a Variant is of VarType vbObject. This could occur if the Variant actually references (or once referenced) an object, or if it contains Nothing.
IsObject returns True if identifier is a variable declared with Object type or any valid class type, or if identifier is a Variant of VarType vbObject, or a user-defined object; otherwise, it returns False. IsObject returns True even if the variable has been set to Nothing.
Use error trapping to be sure that an object reference is valid.