Plugin Fail to function

The Plugin is doing well in normal product setting but has return to fail if the product is set to variable product

the message has return like this
https://mydomain.com/?wc-api=wc_senangpay_gateway?&status_id=0&order_id=34&transaction_id=&msg=Amount_is_not_a_number&hash=ce95ec8ffbb2ecf6fb2303c93b55f38a

and the screen shown -1 value

please modify this bugs