Laravel Eloquent belongsToMany() with string pk converted to 0

Trying to work through an issue with belongsToMany() where my pk is a string (similar to uuid)

simple products <-- product_productgroups --> productgroups

but products primary key on products is apk varchar(50)

when I use integer values, everything is fine, but if I use strings, they are converted to 0 when searching the pivot table:

$product = App\Products::with('productgroups')->find('B00AE38KM');

"select * from `products` where `products`.`apk` = ? limit 1"

array:1 [▼ 0 => "B00AE38KM" ]

"select productgroups.*, product_productgroup.product_apk as pivot_product_apk, product_productgroup.productgroup_id as pivot_productgroup_id from productgroups inner join product_productgroup on productgroups.id = product_productgroup.productgroup_id where product_productgroup.product_apk in (?)"

array:1 [▼ 0 => 0 ]

My Products model has:

    public function productgroups()
    {
        return $this->belongsToMany('App\ProductGroups', 
           'product_productgroup', 
           'product_apk', 
           'productgroup_id');
    }   

TIA!

via Chebli Mohamed