php – laravel errno 150外键约束形成错误

有3个表有2个外键：

Schema::create('users',function (Blueprint $table) {
                    $table->increments('id');
                    $table->string('name');
                    $table->string('email')->unique();
                    $table->string('password');
                    $table->rememberToken();
                    $table->timestamps();
                });

        Schema::create('firms',function (Blueprint $table) {
                    $table->increments('id');
                    $table->string('title')->nullable();  
                    $table->integer('user_id')->unsigned()->nullable();
                    $table->foreign('user_id')->references('id')->on('users');
                    $table->timestamps();
                });
       Schema::create('jobs',function (Blueprint $table) {
            $table->increments('id');
            $table->string('title')->nullable();
            $table->integer('firm_id')->unsigned()->nullable();
            $table->foreign('firm_id')->references('id')->on('firms');
            $table->timestamps();
        });

运行迁移后出错：

[IlluminateDatabaseQueryException]
  SQLSTATE[HY000]: General error: 1005 Can't create table `job`.`#sql-5fc_a1`
   (errno: 150 "Foreign key constraint is incorrectly formed") (SQL: alter ta
  ble `firms` add constraint `firms_user_id_foreign` foreign key (`user_id`)
  references `users` (`id`))

  [PDOException]
  SQLSTATE[HY000]: General error: 1005 Can't create table `job`.`#sql-5fc_a1`
   (errno: 150 "Foreign key constraint is incorrectly formed")