Model not inserting timestamp when using PostgreSQL

Hey all!

I recently started trying to port over to Postgres from MySQL, and have bumped into a rather perplexing issue: My models aren't filling in timestamps.

Here's my relevant migration:

        Schema::create('categories', function(Blueprint $table)
        {
            $table->increments('id');
            $table->string('name')->unique();
            $table->timestamps();
            $table->softDeletes();
        });

        Schema::create('category_nouns', function(Blueprint $table)
        {
            $table->increments('id');
            $table->string('noun');
            $table->integer('weight');
            $table->integer('category_id')->unsigned()->default(0);
            $table->foreign('category_id')->references('id')->on('categories');
            $table->timestamps();
            $table->softDeletes();
        });

And here are my model setups:

class Category extends Model {

    use SoftDeletes;

    protected $timestamps = true;
    protected $guarded = ['id'];
    protected $table ='categories';
    
    /* input requirements */
    public static $rules = ['name'=>'required'];

class CategoryNouns extends Model {

    use SoftDeletes;

    protected $timestamps = true;
    protected $guarded = ['id'];
    protected $table ='category_nouns';

    /* input requirements */
    public static $rules = [
                            'noun'        => 'required|unique:category_nouns',
                            'weight'      => 'required|integer',
                            'category_id' => 'required|integer'
                           ];

(the create methods have not been overwritten)

Here's the code:

        // load file
        $inputCsv = Reader::createFromPath($file);

        //@TODO: Progress Bars @ http://laravel.com/docs/5.1/artisan#writing-output

        // assume headers are categories
        $headers = $inputCsv->fetchOne(0);
        foreach($headers as $category) {
            $cat = Category::firstOrCreate(['name'=>$category]);
            $cats[$category] = $cat->id;
        }

        // load nouns and associate to their respective categories
        $data = $inputCsv->setOffset(1)->fetchAssoc(array_flip($cats)); // skip headers now
        foreach($data as $row) {
            foreach($row as $cat_name=>$noun) {
                $noun_exists = CategoryNouns::where('noun','=',$noun)->first();

                if(!empty($noun) && !isset($noun_exists->exists)) {
                    $cat_noun = CategoryNouns::create(['noun' => $noun, 'weight'=> 1, 'category_id' => $cats[$cat_name]]);
                }
            }
        }
        $this->comment("Done.");

And finally, here's the error I keep getting:

[Illuminate\Database\QueryException]                                                                                                                                    
  SQLSTATE[23502]: Not null violation: 7 ERROR:  null value in column "created_at" violates not-null constraint                                                           
  DETAIL:  Failing row contains (2, Utilities, null, null, null). (SQL: insert into "categories" ("name") values (Utilities) returning "id")

Any help would be appreciated. Thanks in advance!