Updating line item quantities in 2-1-stable admin

[johncox00]

I'm having an issue when trying to update line item quantities in an order in the spree admin. When I edit an order and update the quantity of a line item in the order, I get the following error when clicking the check button to submit the change:

wrong number of arguments (1 for 3)
.../vendor/bundle/ruby/2.1.0/bundler/gems/spree_flexi_variants-eb64955b1844/app/models/spree/order_decorator.rb:36:in `find_line_item_by_variant'

Any thoughts on best way to rectify?