diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 3078bb85179..ad529447fc3 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -3771,11 +3771,13 @@ static void replace_helper(QString &str, QSpan<size_t> indices, qsizetype blen,
     const qsizetype oldSize = str.data_ptr().size;
     const qsizetype adjust = indices.size() * (after.size() - blen);
     const qsizetype newSize = oldSize + adjust;
-    if (str.data_ptr().needsDetach() || needsReallocate(str, newSize)) {
+    if (str.data_ptr().needsDetach()) {
         replace_with_copy(str, indices, blen, after);
         return;
     }
 
+    str.reserve(newSize);
+
     if (QtPrivate::q_points_into_range(after.begin(), str))
         // Copy after if it lies inside our own d.b area (which we could
         // possibly invalidate via a realloc or modify by replacement)