This issue was fixed in Magento Patch # SUPEE-6511
You can apply the patch or you can manually update the file because the change is pretty straightforward.
Open app/code/core/Mage/ImportExport/Model/Export/Entity/Product.php
Just replace the line#875
$dataRow += $stockItemRows[$productId];
with
if (isset($stockItemRows[$productId])) {
$dataRow = array_merge($dataRow, $stockItemRows[$productId]);
}