Strange Index Behaviour Depending on Default

If you run: 

index(
    {"One", "Two", "Three", "Four"},
    {100, 200, 300},
    "Not Possible"
)

you'd get, as expected:

{
    "Not Possible",
    "Not Possible", 
    "Not Posisble"
}

However, if instead you run: 

index(
    {"One", "Two", "Three", "Four"},
    {100, 200, 300},
    {}
)

you'd get: 

{
    "One",
    "Two",
    "Three
}

whereas I would have expected an empty list. Why does this happen?