Aggregation on a scripted field

Hello all,

I have created a scripted field to an index.
As far as I can see it is aggregatable.
See below:

GET master*/_field_caps?fields=Size
{
"fields" : {
"Size" : {
"text" : {
"type" : "text",
"searchable" : true,
"aggregatable" : true
}
}
}
}

But queries with aggregations return no results.

Am I missing something?

Regards Hans