diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 77118ea5d73..2cc55ef0138 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -2755,12 +2755,12 @@ static qsizetype lastIndexOfHelper(const char *haystack, qsizetype l, const char
                                    qsizetype ol, qsizetype from)
 {
     auto delta = l - ol;
-    if (from < 0)
-        from = delta;
-    if (from < 0 || from > l)
+    if (from > l)
         return -1;
-    if (from > delta)
+    if (from < 0 || from > delta)
         from = delta;
+    if (from < 0)
+        return -1;
 
     const char *end = haystack;
     haystack += from;
diff --git a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
index df7e14ef3c7..979b79c6709 100644
--- a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
+++ b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
@@ -2665,6 +2665,9 @@ void tst_QByteArray::indexOf_data()
     QTest::newRow("found_char_star_0x80") << haystack << ByteArrayOrChar("\x80"_ba) << 12 << 0;
     QTest::newRow("found_char_0x80") << haystack << ByteArrayOrChar('\x80') << 12 << 0;
     QTest::newRow("found_char_0x81") << haystack << ByteArrayOrChar('\x81') << 20 << 0;
+
+    // Make the needle sufficiently large to try to potentially trip boundary guards:
+    QTest::newRow("needle_larger_than_haystack") << "b"_ba << ByteArrayOrChar(":"_ba.repeated(4096)) << -1 << 0;
 }
 
 void tst_QByteArray::indexOf()
@@ -2708,6 +2711,9 @@ void tst_QByteArray::lastIndexOf_data()
     QTest::newRow("found_char_star_0x80") << haystack << ByteArrayOrChar("\x80"_ba) << 12 << -1;
     QTest::newRow("found_char_0x80") << haystack << ByteArrayOrChar('\x80') << 12 << -1;
     QTest::newRow("found_char_0x81") << haystack << ByteArrayOrChar('\x81') << 20 << -1;
+
+    // Make the needle sufficiently large to try to potentially trip boundary guards:
+    QTest::newRow("needle_larger_than_haystack") << "b"_ba << ByteArrayOrChar(":"_ba.repeated(4096)) << -1 << 0;
 }
 
 void tst_QByteArray::lastIndexOf()