diff --git a/src/network/access/qnetworkreplywasmimpl.cpp b/src/network/access/qnetworkreplywasmimpl.cpp
index fee7f7a4479..005662de36d 100644
--- a/src/network/access/qnetworkreplywasmimpl.cpp
+++ b/src/network/access/qnetworkreplywasmimpl.cpp
@@ -9,6 +9,7 @@
 #include <QtCore/qcoreapplication.h>
 #include <QtCore/qfileinfo.h>
 #include <QtCore/qthread.h>
+#include <QtCore/private/qtools_p.h>
 
 #include <private/qnetworkaccessmanager_p.h>
 #include <private/qnetworkfile_p.h>
@@ -300,7 +301,7 @@ static int parseHeaderName(const QByteArray &headerName)
     if (headerName.isEmpty())
         return -1;
 
-    switch (tolower(headerName.at(0))) {
+    switch (QtMiscUtils::toAsciiLower(headerName.front())) {
     case 'c':
         if (qstricmp(headerName.constData(), "content-type") == 0)
             return QNetworkRequest::ContentTypeHeader;
diff --git a/src/network/access/qnetworkrequest.cpp b/src/network/access/qnetworkrequest.cpp
index 6a57eaa910a..a513fa318f3 100644
--- a/src/network/access/qnetworkrequest.cpp
+++ b/src/network/access/qnetworkrequest.cpp
@@ -13,6 +13,7 @@
 #include "QtCore/qshareddata.h"
 #include "QtCore/qlocale.h"
 #include "QtCore/qdatetime.h"
+#include "QtCore/private/qtools_p.h"
 
 #include <ctype.h>
 #if QT_CONFIG(datestring)
@@ -1094,7 +1095,7 @@ static int parseHeaderName(const QByteArray &headerName)
     if (headerName.isEmpty())
         return -1;
 
-    switch (tolower(headerName.at(0))) {
+    switch (QtMiscUtils::toAsciiLower(headerName.front())) {
     case 'c':
         if (headerName.compare("content-type", Qt::CaseInsensitive) == 0)
             return QNetworkRequest::ContentTypeHeader;