Not sure that I understand what you're reporting here. What call are you sending and what are you considering and invalid value?
We are trying to fetch the Get API's (REST) of Marketo, if we provide the data for which search criteria does not satisfied it displays "success": true. Here we expect the warning message which should indicate the user, for provided search parameters data does not exists and "Success: False"
Method : Get
Now if I provide invalid filterValue, below response is displayed
Same behavior is also observed in other objects, like Get Lead Changes http://developers.marketo.com/documentation/rest/get-lead-changes/ (Details are give in above thread)
This is the expected behavior. We do not validate filterValues, we perform a match against them. If there are no matches, the call was still a success, there are simply no results to return.