Clean installation of zencart 1.5.6a.
I'm getting an error while creating a coupon for free shipping only. Discount coupons for certain amount or percentage discount can be created just fine.

Blank page with the following error displays after confirmation
WARNING: An Error occurred, please refresh the page and try again.If you were entering information, press the BACK button in your browser and re-check the information you had entered to be sure you left no blank fields.

I've tried filling in all fields and I get the same error.

Log file
[B][21-May-2019 10:41:29 America/Detroit] Request URI: /... ADMIN .../coupon_admin.php?action=update_confirm&oldaction=new&cid=1&page=1, IP address:
#1 trigger_error() called at [/... CATALOG .../includes/classes/db/mysql/query_factory.php:171]
#2 queryFactory->show_error() called at [/... CATALOG .../includes/classes/db/mysql/query_factory.php:143]
#3 queryFactory->set_error() called at [/... CATALOG .../includes/classes/db/mysql/query_factory.php:270]
#4 queryFactory->Execute() called at [/... ADMIN .../includes/functions/database.php:51]
#5 zen_db_perform() called at [/... ADMIN .../coupon_admin.php:445]
--> PHP Fatal error: 1366:Incorrect decimal value: '' for column `xxxxx`.`coupons`.`coupon_amount` at row 1 :: insert into coupons (coupon_code, coupon_amount, coupon_product_count, coupon_type, uses_per_coupon, uses_per_user, coupon_minimum_order, restrict_to_products, restrict_to_categories, coupon_start_date, coupon_expire_date, date_created, date_modified, coupon_zone_restriction, coupon_calc_base, coupon_order_limit, coupon_is_valid_for_sales, coupon_active) values ('FREESHIPPING', '', '0', 'S', '100000000', '100000000', '100', '', '', '2019-05-21', '2020-05-21', now(), now(), '5', '0', '0', '1', 'Y') ==> (as called by) /... ADMIN .../includes/functions/database.php on line 51 <== in /... CATALOG .../includes/classes/db/mysql/query_factory.php on line 171.

Any idea what could be happening??