diff --git a/src/main/java/kyu4/RomanNumerals.java b/src/main/java/kyu4/RomanNumerals.java
index 3dbf316..4a7c5f4 100644
--- a/src/main/java/kyu4/RomanNumerals.java
+++ b/src/main/java/kyu4/RomanNumerals.java
@@ -84,15 +84,22 @@ public static String toRoman(int n) {
     }
 
     public static int fromRoman(String romanNumeral) {
+        if (romanNumeral == null) {
+            throw new IllegalArgumentException("Roman numeral must not be null");
+        }
+
         // Traverse from right to left: add when symbol value is >= previous,
         // subtract otherwise. This naturally applies the subtractive rule.
         StringBuilder stringBuilderReverse = new StringBuilder(romanNumeral).reverse();
         char[] chars = stringBuilderReverse.toString().toCharArray();
-        int result = 0;
+        long result = 0;
         int previous = 0;
         for (char c : chars
         ) {
             Integer integer = MAP_FROM.get(c);
+            if (integer == null) {
+                throw new IllegalArgumentException("Unsupported Roman numeral character: " + c);
+            }
             if (integer >= previous) {
                 result += integer;
             } else {
@@ -101,7 +108,7 @@ public static int fromRoman(String romanNumeral) {
             previous = integer;
         }
 
-        return result;
+        return Math.toIntExact(result);
     }
 
 
diff --git a/src/test/java/kyu4/RomanNumeralsTest.java b/src/test/java/kyu4/RomanNumeralsTest.java
index b2f2904..1685c26 100644
--- a/src/test/java/kyu4/RomanNumeralsTest.java
+++ b/src/test/java/kyu4/RomanNumeralsTest.java
@@ -1,9 +1,11 @@
 package kyu4;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.stream.Stream;
 import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
@@ -30,6 +32,19 @@ void shouldRoundTripCanonicalRomanNumerals(int value) {
         assertEquals(value, RomanNumerals.fromRoman(RomanNumerals.toRoman(value)));
     }
 
+    @ParameterizedTest
+    @MethodSource("invalidRomanNumerals")
+    void shouldRejectInvalidRomanNumerals(String romanNumeral) {
+        assertThrows(IllegalArgumentException.class, () -> RomanNumerals.fromRoman(romanNumeral));
+    }
+
+    @Test
+    void shouldRejectResultsOutsideIntegerRange() {
+        String romanNumeral = "M".repeat(2_147_484);
+
+        assertThrows(ArithmeticException.class, () -> RomanNumerals.fromRoman(romanNumeral));
+    }
+
     private static Stream<Arguments> romanCases() {
         return Stream.of(
                 Arguments.of(1, "I"),
@@ -45,4 +60,8 @@ private static Stream<Arguments> romanCases() {
                 Arguments.of(3_999, "MMMCMXCIX")
         );
     }
+
+    private static Stream<String> invalidRomanNumerals() {
+        return Stream.of((String) null, "A", "MCMX?");
+    }
 }