Hi,
I think i’ve bumped into a calculation bug. Subtotal should be 0.
Looks that way, we’ll make sure this is fixed in our next release.
I think i’ve found another bug.
Go to Settings > Invoice Settings > Invoice Options
Check “If your line items quantities are always 1, then you can declutter invoices by no longer displaying this field.”
Go to Invoices > Create
Notice how the Quantity column is also missing on the create invoice page.
Based on the checkmark description I should be able to enter quantities, it should just hide the quantity column on the invoice itself when all quantities are 1.
It’s by design, it’s currently all or nothing.
Oh okay, then the checkmark description is a bit off 
Something like this would be more clear:
“Disable the quantities column, then you can declutter invoices by no longer displaying this field.”
Thanks for the suggestion, we’ll make the change.
If you’d like to patch your local install for the subtotal issue you can apply this change.
https://github.com/invoiceninja/invoiceninja/commit/ea5460d91bad2aa5b09012f9c6ac6ea1678e81e3
It will be included with our next release.