Parse failure on orderby

I have indexed my content. I have a field that is an int (my_custom_field below) and would like to sort on it. I'm getting the error below:

Parse Failure [No mapping found for [ContentApiModel.my_custom_field] in order to sort on]];

The API URI looks like this:

'v2.0/search/content/?query=halle&orderby=my_custom_field desc&skip=0&top=10'