diff --git a/target-service/src/main/java/com/example/targetservice/service/OrderService.java b/target-service/src/main/java/com/example/targetservice/service/OrderService.java
index 73659e8..041aca4 100644
--- a/target-service/src/main/java/com/example/targetservice/service/OrderService.java
+++ b/target-service/src/main/java/com/example/targetservice/service/OrderService.java
@@ -5,8 +5,11 @@
 @Service
 public class OrderService {
 
-    /** Calculates unit price without guarding invalid quantities. */
+    /** Calculates unit price, rejecting non-positive quantities. */
     public int calculateUnitPrice(int totalCents, int quantity) {
+        if (quantity <= 0) {
+            throw new IllegalArgumentException("quantity must be positive");
+        }
         return totalCents / quantity;
     }
 }