when we try to create sale, and add certain items, we are getting a message like this Stock not available, but stock available. Another symptom is that when the submit button is clicked to complete the sale, the page just flashes but remains on same Sale screen.
I have enabled development mode and I get the error message below. I have also enabled logging=1 but not errors are shown in the error log.

A PHP Error was encountered
Severity: Warning
Message: Division by zero
Filename: models/Site.php
Line Number: 72
Backtrace:
File: /home/deltexart/Inventory/app/models/Site.php
Line: 72
Function: _error_handler
File: /home/deltexart/Inventory/app/models/Site.php
Line: 923
Function: calculateCost
File: /home/deltexart/Inventory/app/models/Site.php
Line: 262
Function: item_costing
File: /home/deltexart/Inventory/app/models/admin/Sales_model.php
Line: 71
Function: costing
File: /home/deltexart/Inventory/app/controllers/admin/Sales.php
Line: 265
Function: addSale
File: /home/deltexart/Inventory/index.php
Line: 273
Function: require_once
A PHP Error was encountered
Severity: Warning
Message: Division by zero
Filename: models/Site.php
Line Number: 72
Backtrace:
File: /home/deltexart/Inventory/app/models/Site.php
Line: 72
Function: _error_handler
File: /home/deltexart/Inventory/app/models/Site.php
Line: 923
Function: calculateCost
File: /home/deltexart/Inventory/app/models/admin/Sales_model.php
Line: 86
Function: item_costing
File: /home/deltexart/Inventory/app/controllers/admin/Sales.php
Line: 265
Function: addSale
File: /home/deltexart/Inventory/index.php
Line: 273
Function: require_once