Laravel 5: Foreign key constraint is incorrectly formed

Schema::create('orders', function (Blueprint $table) {
   $table->bigIncrements('id');
   $table->primary('id');
   $table->bigInteger('numPedido')->unsigned();
   $table->string('name', 300);
   $table->string('business', 300)->nullable();
   $table->string('dni', 20);
   $table->string('phone', 20);
   $table->string('address', 300);
   $table->string('cp', 10);
   $table->string('town', 100);
   $table->string('city', 100);
   $table->string('email', 300);
   $table->timestamps();
});

Schema::create('order_items', function (Blueprint $table) {
   $table->bigIncrements('id'); 
   $table->primary('id');
   $table->bigInteger('order_numPedido')->unsigned();
   $table->foreign('order_numPedido')
         ->references('numPedido')
         ->on('orders')
         ->onDelete('cascade');
   $table->string('name', 300);
   $table->string('quantity', 5);
   $table->string('import', 10);
});