Error while uploading csv

Hi,

I’m using your plugin for a while now.
I have setup csv import to update stock of products.

I noticed that variable products don’t update stock levels and that I get fatal error when I try to do that.

Detalji gre?ke
===============
Gre?ka vrste E_ERROR je izazvana na redu 856 datoteke /home/royalexp/balavander.com/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-data.php. Poruka gre?ke: Uncaught WC_Data_Exception: Neispravna ili duplirana ?ifra proizvoda. in /home/royalexp/balavander.com/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-data.php:856
Stack trace:
#0 /home/royalexp/balavander.com/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-product.php(822): WC_Data->error('product_invalid...', 'Neispravna ili ...', 400, Array)
#1 /home/royalexp/balavander.com/wp-content/plugins/woocommerce-stock-manager/admin/includes/class-wsm-save.php(97): WC_Product->set_sku('B0002')
#2 /home/royalexp/balavander.com/wp-content/plugins/woocommerce-stock-manager/admin/includes/class-wsm-save.php(29): WSM_Save::save_data(Array, '1958')
#3 /home/royalexp/balavander.com/wp-content/plugins/woocommerce-stock-manager/admin/views/import-export.php(222): WSM_Save::save_one_item(Array, '1958')
#4 /home/royalexp/balavander.com/wp-content/plugins/woocommerce-stock-manager/admin/class-stock-manager-admin.php(305): include_once('/home/royalexp/...')
#5 /home/royalexp/balavander.com/wp-includ

I just exported csv from your plugin, change stock levels and then try to upload it back again.

Can you tell me what may be the solution.
This only happens for variable products.