diff --git a/src/main/java/com/thealgorithms/conversions/HexToOct.java b/src/main/java/com/thealgorithms/conversions/HexToOct.java
index d3a672d37424..ef696d305ee4 100644
--- a/src/main/java/com/thealgorithms/conversions/HexToOct.java
+++ b/src/main/java/com/thealgorithms/conversions/HexToOct.java
@@ -14,6 +14,7 @@ private HexToOct() {
      *
      * @param hex The Hexadecimal number as a String.
      * @return The Decimal equivalent as an integer.
+     * @throws IllegalArgumentException if the string contains a character that is not a hexadecimal digit
      */
     public static int hexToDecimal(String hex) {
         String hexDigits = "0123456789ABCDEF";
@@ -23,6 +24,9 @@ public static int hexToDecimal(String hex) {
         for (int i = 0; i < hex.length(); i++) {
             char hexChar = hex.charAt(i);
             int digitValue = hexDigits.indexOf(hexChar);
+            if (digitValue == -1) {
+                throw new IllegalArgumentException("Invalid hexadecimal digit: " + hexChar);
+            }
             decimalValue = 16 * decimalValue + digitValue;
         }
 
diff --git a/src/test/java/com/thealgorithms/conversions/HexToOctTest.java b/src/test/java/com/thealgorithms/conversions/HexToOctTest.java
index b3f94b19b6e6..5793b640316b 100644
--- a/src/test/java/com/thealgorithms/conversions/HexToOctTest.java
+++ b/src/test/java/com/thealgorithms/conversions/HexToOctTest.java
@@ -1,6 +1,7 @@
 package com.thealgorithms.conversions;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
@@ -30,4 +31,11 @@ public void testHexToOctal() {
         assertEquals(0, HexToOct.hexToOctal("0"));
         assertEquals(7777, HexToOct.hexToOctal("FFF"));
     }
+
+    @Test
+    public void testInvalidHexDigitThrows() {
+        assertThrows(IllegalArgumentException.class, () -> HexToOct.hexToDecimal("1G"));
+        assertThrows(IllegalArgumentException.class, () -> HexToOct.hexToDecimal("0x1A"));
+        assertThrows(IllegalArgumentException.class, () -> HexToOct.hexToOctal("Z"));
+    }
 }