diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 7fc783cea5..70526825f3 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -2506,6 +2506,8 @@ static qsizetype lastIndexOfHelper(const char *haystack, qsizetype l, const char
 
 static inline qsizetype lastIndexOfCharHelper(QByteArrayView haystack, qsizetype from, char needle) noexcept
 {
+    if (haystack.size() == 0)
+        return -1;
     if (from < 0)
         from += haystack.size();
     else if (from > haystack.size())
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 0ca5c7a2cb..640c1e4ee5 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -10344,10 +10344,12 @@ template <typename Haystack>
 static inline qsizetype qLastIndexOf(Haystack haystack, QChar needle,
                                      qsizetype from, Qt::CaseSensitivity cs) noexcept
 {
+    if (haystack.size() == 0)
+        return -1;
     if (from < 0)
         from += haystack.size();
-    if (std::size_t(from) >= std::size_t(haystack.size()))
-        return -1;
+    else if (std::size_t(from) > std::size_t(haystack.size()))
+        from = haystack.size() - 1;
     if (from >= 0) {
         char16_t c = needle.unicode();
         const auto b = haystack.data();
diff --git a/tests/auto/corelib/text/qstring/tst_qstring.cpp b/tests/auto/corelib/text/qstring/tst_qstring.cpp
index 7ad218be96..c2a7b93c7d 100644
--- a/tests/auto/corelib/text/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/text/qstring/tst_qstring.cpp
@@ -1723,7 +1723,7 @@ void tst_QString::lastIndexOf_data()
 
     QTest::newRow("10") << a << "G" <<  -1 <<  int(a.size())-1 << true;
     QTest::newRow("11") << a << "G" <<  int(a.size())-1 <<  int(a.size())-1 << true;
-    QTest::newRow("12") << a << "G" <<  int(a.size()) <<  -1 << true;
+    QTest::newRow("12") << a << "G" <<  int(a.size()) <<  int(a.size())-1 << true;
     QTest::newRow("13") << a << "A" <<  0 <<  0 << true;
     QTest::newRow("14") << a << "A" <<  -1*int(a.size()) <<  0 << true;
 
diff --git a/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp b/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp
index 88ccc8d3bf..3447af88eb 100644
--- a/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp
+++ b/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp
@@ -2561,7 +2561,7 @@ void tst_QStringApiSymmetry::lastIndexOf_data(bool rhsHasVariableLength)
     }
 
 #define ROW(h, n, st, cs, cis) \
-    QTest::addRow("haystack: %s, needle: %s", #h, #n) << h << QLatin1String(#h) \
+    QTest::addRow("haystack: %s, needle: %s, start %d", #h, #n, st) << h << QLatin1String(#h) \
                                        << n << QLatin1String(#n) \
                                        << qsizetype(st) << qsizetype(cs) << qsizetype(cis)
 
@@ -2578,8 +2578,8 @@ void tst_QStringApiSymmetry::lastIndexOf_data(bool rhsHasVariableLength)
     ROW(ABCDEFGHIEfGEFG, g,  14, -1, 14);
     ROW(ABCDEFGHIEfGEFG, G,  13, 11, 11);
     ROW(ABCDEFGHIEfGEFG, g,  13, -1, 11);
-    ROW(ABCDEFGHIEfGEFG, G,  15, -1, -1);
-    ROW(ABCDEFGHIEfGEFG, g,  15, -1, -1);
+    ROW(ABCDEFGHIEfGEFG, G,  15, 14, 14);
+    ROW(ABCDEFGHIEfGEFG, g,  15, -1, 14);
     ROW(ABCDEFGHIEfGEFG, B,  14,  1,  1);
     ROW(ABCDEFGHIEfGEFG, b,  14, -1,  1);
     ROW(ABCDEFGHIEfGEFG, B,  -1,  1,  1);