"e.index is not a function" for handles in a directive in 1.3.8

[peterjhart]

ng-sortable throws an error when the as-sortable-item-handle is inside a directive (in my case, with an isolate scope).

The structure is basically:
Parent Template

<div as-sortable="vm.sortableConfig" ng-model="vm.things">
    <my-directive ng-repeat="thing in vm.things track by thing.id" as-sortable-item thing="thing"></my-directive as-sortable-item>
</div>

Directive Template

<div as-sortable-item-handle>
  {{thing.title}}
</div>

The error (I removed file/lines since it's concatenated):

Uncaught TypeError: e.index is not a function
    at Object.dragItem
    at _
    at HTMLDocument.a
    at nt
    at HTMLDocument.n
dragItem
_
a
nt
n

When I revert to 1.3.7, this works. When I upgrade to 1.3.8, I get the error.

Google Chrome, AngularJS 1.6.4

[mikey0000]

#393