diff --git a/src/main/java/algorithms/sprint2/Calculator.java b/src/main/java/algorithms/sprint2/Calculator.java
index 5f1da48..0a5ad1c 100644
--- a/src/main/java/algorithms/sprint2/Calculator.java
+++ b/src/main/java/algorithms/sprint2/Calculator.java
@@ -45,9 +45,16 @@ private static int eval(FastIn in) throws IOException {
             if (t.length() == 1) {
                 char op = t.charAt(0);
                 if (op == '+' || op == '-' || op == '*' || op == '/') {
+                    if (st.size() < 2) {
+                        throw new IllegalArgumentException("Operator requires two operands: " + op);
+                    }
                     int b = st.pop();
                     int a = st.pop();
 
+                    if (op == '/' && b == 0) {
+                        throw new IllegalArgumentException("Division by zero");
+                    }
+
                     int r;
                     if (op == '+') {
                         r = a + b;
@@ -64,9 +71,16 @@ private static int eval(FastIn in) throws IOException {
                 }
             }
 
-            st.push(parseInt(t));
+            try {
+                st.push(parseInt(t));
+            } catch (NumberFormatException exception) {
+                throw new IllegalArgumentException("Invalid token: " + t, exception);
+            }
         }
 
+        if (st.size() != 1) {
+            throw new IllegalArgumentException("Expression must produce exactly one result");
+        }
         return st.peek();
     }
 
@@ -74,7 +88,13 @@ private static void run() throws Exception {
         FastIn in = new FastIn(System.in);
         FastOut out = new FastOut(System.out);
 
-        int ans = eval(in);
+        final int ans;
+        try {
+            ans = eval(in);
+        } catch (IllegalArgumentException exception) {
+            System.err.println("Invalid expression: " + exception.getMessage());
+            return;
+        }
 
         out.writeInt(ans);
         out.writeByte('\n');
diff --git a/src/test/java/algorithms/AlgorithmCliTest.java b/src/test/java/algorithms/AlgorithmCliTest.java
index 0da7388..4dc06af 100644
--- a/src/test/java/algorithms/AlgorithmCliTest.java
+++ b/src/test/java/algorithms/AlgorithmCliTest.java
@@ -18,6 +18,7 @@
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
+import org.junit.jupiter.params.provider.ValueSource;
 
 @Tag("integration")
 class AlgorithmCliTest {
@@ -50,6 +51,12 @@ void distancesRunReadsAndWritesContestFiles() throws Exception {
         }
     }
 
+    @ParameterizedTest
+    @ValueSource(strings = {"+", "", "1 0 /", "abc", "1 2"})
+    void calculatorRejectsMalformedExpressionsWithoutThrowing(String input) throws Exception {
+        assertEquals("", invokeWithStdio("algorithms.sprint2.Calculator", "run", input));
+    }
+
     private static Stream<Arguments> stdioCases() {
         return Stream.of(
                 arguments("algorithms.sprint0.SlidingAverage", "main",