diff --git a/src/corelib/text/qlocale.cpp b/src/corelib/text/qlocale.cpp
index d240494018..eb180388a1 100644
--- a/src/corelib/text/qlocale.cpp
+++ b/src/corelib/text/qlocale.cpp
@@ -3565,7 +3565,7 @@ QString QCalendarBackend::dateTimeToString(QStringView format, const QDateTime &
             }
         }
         if (!used)
-            result.append(QString(repeat, c));
+            result.resize(result.size() + repeat, c);
         i += repeat;
     }