diff --git a/src/main/java/org/apache/commons/text/StrBuilder.java b/src/main/java/org/apache/commons/text/StrBuilder.java
index fce069d001..f1df60a2a2 100644
--- a/src/main/java/org/apache/commons/text/StrBuilder.java
+++ b/src/main/java/org/apache/commons/text/StrBuilder.java
@@ -376,7 +376,7 @@ public StrBuilder append(final char[] chars, final int startIndex, final int len
             return appendNull();
         }
         if (startIndex < 0 || startIndex > chars.length) {
-            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + length);
+            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + startIndex);
         }
         if (length < 0 || startIndex + length > chars.length) {
             throw new StringIndexOutOfBoundsException("Invalid length: " + length);
diff --git a/src/main/java/org/apache/commons/text/TextStringBuilder.java b/src/main/java/org/apache/commons/text/TextStringBuilder.java
index b3ea8585f7..2977fa3232 100644
--- a/src/main/java/org/apache/commons/text/TextStringBuilder.java
+++ b/src/main/java/org/apache/commons/text/TextStringBuilder.java
@@ -483,7 +483,7 @@ public TextStringBuilder append(final char[] chars, final int startIndex, final
             return appendNull();
         }
         if (startIndex < 0 || startIndex > chars.length) {
-            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + length);
+            throw new StringIndexOutOfBoundsException("Invalid startIndex: " + startIndex);
         }
         if (length < 0 || startIndex + length > chars.length) {
             throw new StringIndexOutOfBoundsException("Invalid length: " + length);
diff --git a/src/test/java/org/apache/commons/text/StrBuilderTest.java b/src/test/java/org/apache/commons/text/StrBuilderTest.java
index 6ae331e931..5edfec6fc6 100644
--- a/src/test/java/org/apache/commons/text/StrBuilderTest.java
+++ b/src/test/java/org/apache/commons/text/StrBuilderTest.java
@@ -2017,4 +2017,21 @@ void testTrim() {
         assertEquals("a b c", sb.trim().toString());
     }
 
+    @Test
+    void testErrorMessageShowsCorrectVariable() {
+        final StrBuilder sb = new StrBuilder("Hello");
+        final char[] chars = {'a', 'b', 'c'};
+
+        StringIndexOutOfBoundsException ex = assertThrows(
+                StringIndexOutOfBoundsException.class,
+                () -> sb.append(chars, 1, 4)
+        );
+        assertTrue(ex.getMessage().contains("length: 4"));
+
+        ex = assertThrows(
+                StringIndexOutOfBoundsException.class,
+                () -> sb.append(chars, 7, 3)
+        );
+        assertTrue(ex.getMessage().contains("startIndex: 7"));
+    }
 }
diff --git a/src/test/java/org/apache/commons/text/TextStringBuilderTest.java b/src/test/java/org/apache/commons/text/TextStringBuilderTest.java
index 1900c19d68..1efbf25113 100644
--- a/src/test/java/org/apache/commons/text/TextStringBuilderTest.java
+++ b/src/test/java/org/apache/commons/text/TextStringBuilderTest.java
@@ -2393,4 +2393,21 @@ void testWrap_CharArray_Int_Exceptions() {
         assertThrows(IllegalArgumentException.class, () -> TextStringBuilder.wrap(ArrayUtils.EMPTY_CHAR_ARRAY, 1));
     }
 
+    @Test
+    void testErrorMessageShowsCorrectVariable() {
+        final TextStringBuilder sb = new TextStringBuilder("Hello");
+        final char[] chars = {'a', 'b', 'c'};
+
+        StringIndexOutOfBoundsException ex = assertThrows(
+                StringIndexOutOfBoundsException.class,
+                () -> sb.append(chars, 1, 4)
+        );
+        assertTrue(ex.getMessage().contains("length: 4"));
+
+        ex = assertThrows(
+                StringIndexOutOfBoundsException.class,
+                () -> sb.append(chars, 7, 3)
+        );
+        assertTrue(ex.getMessage().contains("startIndex: 7"));
+    }
 }