diff --git a/core-kotlin/src/test/kotlin/com/baeldung/random/RandomStringUnitTest.kt b/core-kotlin/src/test/kotlin/com/baeldung/random/RandomStringUnitTest.kt
index 74085367e8..62e8dfe720 100644
--- a/core-kotlin/src/test/kotlin/com/baeldung/random/RandomStringUnitTest.kt
+++ b/core-kotlin/src/test/kotlin/com/baeldung/random/RandomStringUnitTest.kt
@@ -52,7 +52,7 @@ class RandomStringUnitTest {
         random.nextBytes(bytes)
 
         var randomString = (0..bytes.size - 1).map { i ->
-            charPool.get((bytes[i] and 0xFF.toByte() and charPool.size.toByte()).toInt())
+            charPool.get((bytes[i] and 0xFF.toByte() and (charPool.size-1).toByte()).toInt())
         }.joinToString("")
 
         assert(randomString.matches(Regex(ALPHANUMERIC_REGEX)))