diff --git a/app/Http/Resources/PlantHarvestResource.php b/app/Http/Resources/PlantHarvestResource.php
index 88cbaa2..69d0b3b 100644
--- a/app/Http/Resources/PlantHarvestResource.php
+++ b/app/Http/Resources/PlantHarvestResource.php
@@ -21,7 +21,7 @@ class PlantHarvestResource extends JsonResource
             'director' => $this->director,
             'area' => $this->area,
             'output' => $this->output,
-            'harvest_at' => $this->harvest_at ? $this->harvest_at->timestamp : '',
+            'harvest_at' => $this->harvest_at ? $this->harvest_at->timestamp : null,
         ];
     }
 }
diff --git a/app/Http/Resources/RegionPlantResource.php b/app/Http/Resources/RegionPlantResource.php
index 9ec2393..ad6b1d5 100644
--- a/app/Http/Resources/RegionPlantResource.php
+++ b/app/Http/Resources/RegionPlantResource.php
@@ -19,8 +19,8 @@ class RegionPlantResource extends JsonResource
             'plant_name' => $this->plant_name,
             'director' => $this->director,
             'area' => $this->area,
-            'start_at' => $this->start_at ? $this->start_at->timestamp : '',
-            'end_at' => $this->end_at ? $this->end_at->timestamp : '',
+            'start_at' => $this->start_at ? $this->start_at->timestamp : null,
+            'end_at' => $this->end_at ? $this->end_at->timestamp : null,
             'plant_state' => $this->plant_state,
         ];
     }