When the merger took place looks like when your brokerage did not keep track correctly.
Normally they will leave the cost basis blank versus a $0 and say cost basis not reported to IRS and let the customer fill it in.
Regardless if they did this or not, you need to put in the correct cost basis and get rid of the $0.
You would then submit the return just like normal, however keep detailed records to verify the cost basis should IRS ask you to verify it. It will do no good to attach this to the return when filing it.
Great that you caught this.
