Laravel5.5. migrate 提示外键约束错误？

In Connection.php line 664:

SQLSTATE[HY000]: General error: 1005 Can't create table n_fruit.#sql-3fa0_23 (errno: 150 "Foreign key constrain
t is incorrectly formed") (SQL: alter table posts add constraint posts_comment_id_foreign foreign key (comment _id) references comments (id) on delete cascade)

In Connection.php line 458:

SQLSTATE[HY000]: General error: 1005 Can't create table n_fruit.#sql-3fa0_23 (errno: 150 "Foreign key constrain
t is incorrectly formed")

Process finished with exit code 1 at 13:53:34.
Execution time: 12,000 ms.

请问给位朋友这要怎么处理，下面是表字段设置
posts表
$table->integer('comment_id')->unsigned()->comment('评论id');
$table->integer('category_id')->unsigned()->comment('所属分类id');
$table->foreign('comment_id')
->references('id')
->on('comments')
->onDelete('cascade');

        $table->foreign('category_id')
            ->references('id')->on('categorys')
            ->onDelete('cascade');
        $table->timestamps();

comments表是存在的