diff --git a/src/java/org/apache/commons/lang/text/StrBuilder.java b/src/java/org/apache/commons/lang/text/StrBuilder.java
index 13e8a6b2d..2d52da538 100644
--- a/src/java/org/apache/commons/lang/text/StrBuilder.java
+++ b/src/java/org/apache/commons/lang/text/StrBuilder.java
@@ -556,10 +556,10 @@ public class StrBuilder implements Cloneable {
             return this;
         }
         if (startIndex < 0 || startIndex > chars.length) {
-            throw new StringIndexOutOfBoundsException("startIndex must be valid");
+            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + length);
         }
         if (length < 0 || (startIndex + length) > chars.length) {
-            throw new StringIndexOutOfBoundsException("length must be valid");
+            throw new StringIndexOutOfBoundsException("Invalid length: " + length);
         }
         if (length > 0) {
             int len = length();