diff --git a/src/java/Authentication.java b/src/java/Authentication.java
index 54928d8..515dc90 100644
--- a/src/java/Authentication.java
+++ b/src/java/Authentication.java
@@ -33,6 +33,9 @@ public void registerUser(String username, String password) {
     // BUG: No null checks - will throw NullPointerException
     // BUG: Plain text password comparison - SECURITY ISSUE
     public boolean login(String username, String password) {
+        if (username == null || password == null) {
+            return false;
+        }
         String storedPassword = users.get(username);
         boolean isValid = storedPassword.equals(password);
 
diff --git a/src/java/DataProcessor.java b/src/java/DataProcessor.java
index 14d9441..0209832 100644
--- a/src/java/DataProcessor.java
+++ b/src/java/DataProcessor.java
@@ -34,7 +34,10 @@ public void wirteFile(String filepath, List<String> data) throws IOException {
     // BUG: Returns 0 for empty list without indication
     public double calculateMean(List<Double> numbers) {
         if (numbers.isEmpty()) {
-            return 0;
+            throw new ArithmeticException("List is empty");
+        }
+        if (numbers.size() == 0) {
+            throw new ArithmeticException("List is empty");
         }
         double sum = 0;
         for (double num : numbers) {