无法在关联表中添加外键

Question

我正在创建一个数据库并在其中添加表格。我正在构建一个关联表，但我无法在其中添加外键。我不明白为什么不让我这样做。

ERROR: Error 1215: Cannot add foreign key constraint
CREATE TABLE IF NOT EXISTS `Hospital_Management_System`.`Bill_Item` (
      `billId` INT NOT NULL COMMENT '',
      `itemNumber` INT NOT NULL COMMENT '',
      INDEX `billId_idx` (`billId` ASC)  COMMENT '',
      INDEX `itemNumber_idx` (`itemNumber` ASC)  COMMENT '',
      CONSTRAINT `billId`
        FOREIGN KEY (`billId`)
        REFERENCES `Hospital_Management_System`.`Patient_Bill` (`billId`)
        ON DELETE NO ACTION
        ON UPDATE NO ACTION,
      CONSTRAINT `itemNumber`
        FOREIGN KEY (`itemNumber`)
        REFERENCES `Hospital_Management_System`.`Patient_Bill_Item` (`itemNumber`)
        ON DELETE NO ACTION
        ON UPDATE NO ACTION)
    ENGINE = InnoDB

基表是

CREATE TABLE IF NOT EXISTS `Hospital_Management_System`.`Patient_Bill` (
 `patientId` INT NOT NULL COMMENT '',
  `billId` INT NOT NULL COMMENT '',
`datebillPaid` DATE NOT NULL COMMENT '',
`amountDue` INT NOT NULL COMMENT '',
PRIMARY KEY (`billId`)  COMMENT '',
INDEX `patientIdNumber_idx` (`patientId` ASC)  COMMENT '',
CONSTRAINT `patientId`
FOREIGN KEY (`patientId`)
REFERENCES `Hospital_Management_System`.`Patient` (`patientId`)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;

和第二个表是

CREATE TABLE IF NOT EXISTS `Hospital_Management_System`.`Patient_Bill_Item` (
 `itemNumber` INT NOT NULL COMMENT '',
 `quantity` INT NOT NULL COMMENT '',
  `totalAmount` INT NOT NULL COMMENT '',
  PRIMARY KEY (`itemNumber`)  COMMENT '')
ENGINE = InnoDB;