diff --git a/src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java b/src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
index 629aab424bd..6fa79baccb9 100644
--- a/src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
+++ b/src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
@@ -1107,10 +1107,9 @@ public final class StringConcatFactory {
         MethodHandle mh = MethodHandles.dropArguments(newString(), 2, ttypes);
 
         long initialLengthCoder = INITIAL_CODER;
-        String lastFragment = "";
         pos = 0;
         for (String fragment : fragments) {
-            lastFragment = fragment;
+            initialLengthCoder = JLA.stringConcatMix(initialLengthCoder, fragment);
 
             if (ttypes.length <= pos) {
                 break;
@@ -1118,14 +1117,15 @@ public final class StringConcatFactory {
 
             Class<?> ttype = ttypes[pos];
             // (long,byte[],ttype) -> long
-            MethodHandle prepender = prepender(lastFragment.isEmpty() ? null : fragment, ttype);
-            initialLengthCoder = JLA.stringConcatMix(initialLengthCoder, fragment);
+            MethodHandle prepender = prepender(fragment.isEmpty() ? null : fragment, ttype);
             // (byte[],long,ttypes...) -> String (unchanged)
             mh = MethodHandles.filterArgumentsWithCombiner(mh, 1, prepender,1, 0, 2 + pos);
 
             pos++;
         }
 
+        String lastFragment = fragments.getLast();
+        initialLengthCoder -= lastFragment.length();
         MethodHandle newArrayCombinator = lastFragment.isEmpty() ? newArray() :
                 newArrayWithSuffix(lastFragment);
         // (long,ttypes...) -> String
diff --git a/test/jdk/java/lang/template/T8313809.java b/test/jdk/java/lang/template/T8313809.java
new file mode 100644
index 00000000000..b673b2b9c44
--- /dev/null
+++ b/test/jdk/java/lang/template/T8313809.java
@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8313809
+ * @summary String template fails with java.lang.StringIndexOutOfBoundsException if last fragment is UTF16
+.
+ * @enablePreview true
+ */
+
+import static java.util.FormatProcessor.FMT;
+
+public class T8313809 {
+    public static void main(final String[] args) throws Exception {
+        double sum = 12.34;
+        final String message = FMT."The sum is : %f\{sum} €"; // this fails
+        if (!message.equals("The sum is : 12.340000 €")) {
+            throw new RuntimeException("Incorrect result");
+        }
+    }
+}
+