diff --git a/smack-core/src/main/java/org/jivesoftware/smack/util/StringUtils.java b/smack-core/src/main/java/org/jivesoftware/smack/util/StringUtils.java
index c2bb9210a..1a4d3a263 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/util/StringUtils.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/util/StringUtils.java
@@ -285,7 +285,7 @@ public class StringUtils {
      * array index.
      */
     private static final char[] numbersAndLetters = ("0123456789abcdefghijklmnopqrstuvwxyz" +
-                    "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
+                    "ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
 
     /**
      * Returns a random String of numbers and letters (lower and upper case)