diff --git a/src/DataTransferObjects/Schemas/FulfillmentInbound/PartneredSmallParcelPackageOutputSchema.php b/src/DataTransferObjects/Schemas/FulfillmentInbound/PartneredSmallParcelPackageOutputSchema.php
index bde5db28..4ecab71f 100644
--- a/src/DataTransferObjects/Schemas/FulfillmentInbound/PartneredSmallParcelPackageOutputSchema.php
+++ b/src/DataTransferObjects/Schemas/FulfillmentInbound/PartneredSmallParcelPackageOutputSchema.php
@@ -16,7 +16,7 @@ class PartneredSmallParcelPackageOutputSchema extends DataTransferObject
 
     public string $carrier_name;
 
-    public string $tracking_id;
+    public ?string $tracking_id;
 
     #[StringEnumValidator(AmazonEnums::PACKAGE_STATUSES)]
     public string $package_status;
diff --git a/tests/Unit/Resources/FulfillmentInboundResourceTest.php b/tests/Unit/Resources/FulfillmentInboundResourceTest.php
index 4ad12404..8a128596 100644
--- a/tests/Unit/Resources/FulfillmentInboundResourceTest.php
+++ b/tests/Unit/Resources/FulfillmentInboundResourceTest.php
@@ -263,6 +263,26 @@ public function testGetTransportDetails_Issue3_CarrierNameNull()
         });
     }
 
+    public function testGetTransportDetails_Issue6_TrackingIDNull()
+    {
+        list($config, $http) = $this->setupConfigWithFakeHttp('fulfillment-inbound/issues/issue-6-tracking-id-null');
+
+        $shipment_id = Str::random();
+
+        $amzn = new AmznSPA($config);
+        $amzn = $amzn->usingMarketplace('ATVPDKIKX0DER');
+        $response = $amzn->fulfillment_inbound->getTransportDetails($shipment_id);
+
+        $this->assertInstanceOf(GetTransportDetailsResponse::class, $response);
+
+        $http->assertSent(function (Request $request) use ($shipment_id) {
+            $this->assertEquals('GET', $request->method());
+            $this->assertEquals('https://sellingpartnerapi-na.amazon.com/fba/inbound/v0/shipments/'.$shipment_id.'/transport', urldecode($request->url()));
+
+            return true;
+        });
+    }
+
     public function testPutTransportDetails()
     {
         list($config, $http) = $this->setupConfigWithFakeHttp('fulfillment-inbound/put-transport-details');
diff --git a/tests/stubs/fulfillment-inbound/issues/issue-6-tracking-id-null.json b/tests/stubs/fulfillment-inbound/issues/issue-6-tracking-id-null.json
new file mode 100644
index 00000000..5f84af96
--- /dev/null
+++ b/tests/stubs/fulfillment-inbound/issues/issue-6-tracking-id-null.json
@@ -0,0 +1,35 @@
+{
+    "payload": {
+        "TransportContent": {
+            "TransportHeader": {
+                "SellerId": "AABBCC",
+                "ShipmentId": "FBAAABBCC",
+                "IsPartnered": true,
+                "ShipmentType": "SP"
+            },
+            "TransportDetails": {
+                "PartneredSmallParcelData": {
+                    "PackageList": [
+                        {
+                            "Dimensions": {
+                                "Length": "5",
+                                "Width": "5",
+                                "Height": "5",
+                                "Unit": "CM"
+                            },
+                            "Weight": {
+                                "Value": "5",
+                                "Unit": "kilograms"
+                            },
+                            "CarrierName": "UNITED_PARCEL_SERVICE_INC",
+                            "PackageStatus": "SHIPPED"
+                        }
+                    ]
+                }
+            },
+            "TransportResult": {
+                "TransportStatus": "ERROR_ON_ESTIMATING"
+            }
+        }
+    }
+}
\ No newline at end of file