Getting no output when using json_encode on an array

My $output array looks like this

Array
(
[0] => Array
    (
        [country] => Abkhazia
        [capital] => Sukhumi
    )

[1] => Array
    (
        [country] => Afghanistan
        [capital] => Kabul
    )

[2] => Array
    (
        [country] => Akrotiri and Dhekelia
        [capital] => Episkopi Cantonment
    )

[3] => Array
    (
        [country] => Albania
        [capital] => Tirana
    )
)

When I echo json_encode($output) i get no output but if I do json_encode($output[0]), I get the first array inside of the $output array. What am I overlooking here?