cannot drop column with foreign key constraint

CREATE TABLE `tab1` (

  `tab1id` int(10) unsigned NOT NULL AUTO_INCREMENT,

  `tab1desc` varchar(60) NOT NULL,

  PRIMARY KEY (`tab1id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

CREATE TABLE `tab2` (

  `tab2id` int(10) unsigned NOT NULL AUTO_INCREMENT,

  `tab1id` int(10) unsigned DEFAULT NULL,

  `tab2desc` varchar(60) NOT NULL,

  PRIMARY KEY (`tab2id`),

  KEY `idxtab2tab1id` (`tab1id`),

  CONSTRAINT `fktab2tab1id` FOREIGN KEY (`tab1id`) REFERENCES `tab1` (`tab1id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

alter table tab2 drop COLUMN tab1id;

Cannot drop index 'idxtab2tab1id': needed in a foreign key constraint