diff --git a/src/main/java/millions/Stock.java b/src/main/java/millions/Stock.java
index ba59d70..41a3ead 100644
--- a/src/main/java/millions/Stock.java
+++ b/src/main/java/millions/Stock.java
@@ -1,6 +1,7 @@
 package millions;
 
 import java.math.BigDecimal;
+import java.util.ArrayList;
 import java.util.List;
 
 public class Stock {
@@ -11,13 +12,13 @@ public class Stock {
   public Stock(String symbol, String company, List<BigDecimal> prices) {
     this.symbol = symbol;
     this.company = company;
-    this.prices = prices;
+    this.prices = new ArrayList<>(prices);
   }
 
   public Stock(String symbol, String company, BigDecimal initialPrice) {
     this.symbol = symbol;
     this.company = company;
-    this.prices = List.of(initialPrice);
+    this.prices = new ArrayList<>(List.of(initialPrice));
   }
 
   public String getSymbol() {