diff --git a/src/BaseModel.php b/src/BaseModel.php
index 25ec4f04..23f1b8cb 100644
--- a/src/BaseModel.php
+++ b/src/BaseModel.php
@@ -72,6 +72,10 @@ public function defineProperty($key, $value)
 
         // Build setter name
         $setterName = "set" . Str::pascal($key);
+	if (!method_exists($this, $setterName)) {
+            // Don't attempt to set new properties which haven't been defined upstream yet
+            return;
+        }
 
         // Calling the setter will type-enforce the values
 
diff --git a/tests/Unit/order.json b/tests/Unit/order.json
index 53c8aef8..913d06da 100644
--- a/tests/Unit/order.json
+++ b/tests/Unit/order.json
@@ -7,6 +7,7 @@
       "@type":"OrderItem",
       "@id":"https://fencing-book/item/12345",
       "orderItemStatus":"https://openactive.io/OrderItemConfirmed",
+      "foo:unknown-property": "Rawr!",
       "unitTaxSpecification":[
         {
           "@type":"TaxChargeSpecification",