Unable to submit multi-select values

I'm trying to submit data to a "multi-select" field type in Silverpop using the following data;

```
[title] => mr
[where_purchased] => Array
    (
        [0] => lidl, 
        [1] => mothercare
        [2] => marks and spencer
    )

[your_date_of_birth] => 2000-01-01
```

But the field is always empty after submission. All other fields are populated correctly. 
