Laravel Relationship method must return an object of type Relation

I've been scouring google trying to figure out a solution. I'm getting the:

ErrorException in Model.php line 2755: Relationship method must return an object of type Illuminate\Database\Eloquent\Relations\Relation

error when I try to go to my posts index page, and a just can't figure out where it's coming from. I can get to the page when I redirect from creating a post, but after that I get the error. Supposedly it's supposed to come from not returning a relationship, but here's my relationships and code:

Post

public function category() 
{
    return $this->belongsTo('App\Category', 'category_id');
}

public function user()
{
    return $this->belongsTo('App\User', 'user_id');
}

User

public function roles() 
{
    return $this->belongsToMany('App\Role')->withTimestamps();
}
public function posts()
{
    return $this->hasMany('App\Post', 'user_id');
}

Role

 public function users() 
{
    return $this->belongsToMany('App\User');
}

Category

public function posts()
{
    return $this->hasMany('App\Post', 'category_id');
}

index.blade.php

@extends('app')

@section('content')
        <h1>Posts</h1>

        @if (Auth::check()) 
                @if (Auth::user()->isAdmin())
                        <a href="/post/create">Create a Post</a>
                @endif
        @endif
        @foreach ($posts as $post)
                @include('includes.post', ['post' => $post, 'link' => true])
        @endforeach
@stop

and post.blade.php

<div class="post">
        <div class="div">
                <div class="postCategory">
                
                </div>
                @if($link)
                        <a href="/post/{{ $post->id }}"><h2>{{ $post->title }}</h2></a>
                @else
                        <h2>{{ $post->title }}</h2>
                @endif
                <p>{!! $post->body !!}</p>
                <p>{{ $post->category->name }}</p>
                <p>Posted by <a href="/user/{{ $post->user->username }}">{{ $post->user->username }}</a> at {{ $post->created_at }}</p>
                
                @if(Auth::check())
                        @if($post->user->username == Auth::user()->username || Auth::user()->isAdmin)
                                {!! Form::open(['method' => 'delete', 'route' => ['post.destroy', $post->id]]) !!}
                                        <input type="submit" value="Delete Post" onclick="return confirm('Are you sure?')"></input>
                                {!! Form::close() !!}
                        @endif
                @endif
                
        </div>
</div>

via Chebli Mohamed