diff --git a/core-java-modules/core-java-lang-5/src/main/java/com/baeldung/convertnumberbases/ConvertNumberBases.java b/core-java-modules/core-java-lang-5/src/main/java/com/baeldung/convertnumberbases/ConvertNumberBases.java
index 405504a965..09b880cd19 100644
--- a/core-java-modules/core-java-lang-5/src/main/java/com/baeldung/convertnumberbases/ConvertNumberBases.java
+++ b/core-java-modules/core-java-lang-5/src/main/java/com/baeldung/convertnumberbases/ConvertNumberBases.java
@@ -2,68 +2,76 @@ package com.baeldung.convertnumberbases;
 
 public class ConvertNumberBases {
 
-    public static String convertNumberToNewBase(String number, int base, int newBase){
+    public static String convertNumberToNewBase(String number, int base, int newBase) {
         return Integer.toString(Integer.parseInt(number, base), newBase);
     }
+
     public static String convertNumberToNewBaseCustom(String num, int base, int newBase) {
         int decimalNumber = convertFromAnyBaseToDecimal(num, base);
-        return convertFromDecimalToBaseX(decimalNumber, newBase);
+        String targetBase = "";
+        try {
+            targetBase = convertFromDecimalToBaseX(decimalNumber, newBase);
+        } catch (IllegalArgumentException e) {
+            e.printStackTrace();
+        }
+        return  targetBase;
     }
 
-    public static String convertFromDecimalToBaseX(int num, int newBase) {
+    public static String convertFromDecimalToBaseX(int num, int newBase) throws IllegalArgumentException {
+        if ((newBase < 2 || newBase > 10) && newBase != 16) {
+            throw new IllegalArgumentException("New base must be from 2 - 10 or 16");
+        }
 
         String result = "";
         int remainder;
         while (num > 0) {
             remainder = num % newBase;
             if (newBase == 16) {
-                if (remainder == 10)
+                if (remainder == 10) {
                     result += 'A';
-                else if (remainder == 11)
+                } else if (remainder == 11) {
                     result += 'B';
-                else if (remainder == 12)
+                } else if (remainder == 12) {
                     result += 'C';
-                else if (remainder == 13)
+                } else if (remainder == 13) {
                     result += 'D';
-                else if (remainder == 14)
+                } else if (remainder == 14) {
                     result += 'E';
-                else if (remainder == 15)
+                } else if (remainder == 15) {
                     result += 'F';
-                else
+                } else {
                     result += remainder;
-            } else
+                }
+            } else {
                 result += remainder;
-
+            }
             num /= newBase;
         }
         return new StringBuffer(result).reverse().toString();
     }
 
     public static int convertFromAnyBaseToDecimal(String num, int base) {
-
-        if (base < 2 || (base > 10 && base != 16))
+        if (base < 2 || (base > 10 && base != 16)) {
             return -1;
-
+        }
         int val = 0;
         int power = 1;
-
         for (int i = num.length() - 1; i >= 0; i--) {
             int digit = charToDecimal(num.charAt(i));
-
-            if (digit < 0 || digit >= base)
+            if (digit < 0 || digit >= base) {
                 return -1;
-
+            }
             val += digit * power;
             power = power * base;
         }
-
         return val;
     }
 
     public static int charToDecimal(char c) {
-        if (c >= '0' && c <= '9')
+        if (c >= '0' && c <= '9') {
             return (int) c - '0';
-        else
+        } else {
             return (int) c - 'A' + 10;
+        }
     }
 }
diff --git a/core-java-modules/core-java-lang-5/src/test/java/com/baeldung/convertnumberbases/ConvertNumberBasesUnitTest.java b/core-java-modules/core-java-lang-5/src/test/java/com/baeldung/convertnumberbases/ConvertNumberBasesUnitTest.java
index 109e1da1b2..a666346b1a 100644
--- a/core-java-modules/core-java-lang-5/src/test/java/com/baeldung/convertnumberbases/ConvertNumberBasesUnitTest.java
+++ b/core-java-modules/core-java-lang-5/src/test/java/com/baeldung/convertnumberbases/ConvertNumberBasesUnitTest.java
@@ -1,5 +1,6 @@
 package com.baeldung.convertnumberbases;
 
+import static com.baeldung.convertnumberbases.ConvertNumberBases.convertFromDecimalToBaseX;
 import static com.baeldung.convertnumberbases.ConvertNumberBases.convertNumberToNewBase;
 import static com.baeldung.convertnumberbases.ConvertNumberBases.convertNumberToNewBaseCustom;
 import static org.junit.jupiter.api.Assertions.*;
@@ -17,4 +18,15 @@ class ConvertNumberBasesUnitTest {
         assertEquals(convertNumberToNewBaseCustom("11001000", 2, 8), "310");
     }
 
+    @Test
+    void whenInputIsOutOfRange_thenIllegalArgumentExceptionIsThrown() {
+        Exception exception = assertThrows(IllegalArgumentException.class, ()-> {
+            convertFromDecimalToBaseX(100, 12);
+        });
+        String expectedMessage = "New base must be from 2 - 10 or 16";
+        String actualMessage = exception.getMessage();
+
+        assertTrue(actualMessage.contains(expectedMessage));
+    }
+
 }
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