diff --git a/src/corelib/io/qurlidna.cpp b/src/corelib/io/qurlidna.cpp
index 9bb6b2a42d..fa0a229f4f 100644
--- a/src/corelib/io/qurlidna.cpp
+++ b/src/corelib/io/qurlidna.cpp
@@ -1502,18 +1502,20 @@ static bool isMappedToNothing(uint uc)
 }
 
 
-static void stripProhibitedOutput(QString *str, int from)
+static bool containsProhibitedOuptut(const QString *str, int from)
 {
-    ushort *out = (ushort *)str->data() + from;
-    const ushort *in = out;
+    const ushort *in = reinterpret_cast<const ushort *>(str->begin() + from);
     const ushort *end = (ushort *)str->data() + str->size();
-    while (in < end) {
+    for ( ; in < end; ++in) {
         uint uc = *in;
         if (QChar(uc).isHighSurrogate() && in < end - 1) {
             ushort low = *(in + 1);
             if (QChar(low).isLowSurrogate()) {
                 ++in;
                 uc = QChar::surrogateToUcs4(uc, low);
+            } else {
+                // unpaired surrogates are prohibited
+                return true;
             }
         }
         if (uc <= 0xFFFF) {
@@ -1538,7 +1540,7 @@ static void stripProhibitedOutput(QString *str, int from)
                 || (uc >= 0xFDD0 && uc <= 0xFDEF)
                 || uc == 0xFEFF
                 || (uc >= 0xFFF9 && uc <= 0xFFFF))) {
-                *out++ = *in;
+                continue;
             }
         } else {
             if (!((uc >= 0x1D173 && uc <= 0x1D17A)
@@ -1562,14 +1564,12 @@ static void stripProhibitedOutput(QString *str, int from)
                 || (uc >= 0xFFFFE && uc <= 0xFFFFF)
                 || (uc >= 0x100000 && uc <= 0x10FFFD)
                 || (uc >= 0x10FFFE && uc <= 0x10FFFF))) {
-                *out++ = QChar::highSurrogate(uc);
-                *out++ = QChar::lowSurrogate(uc);
+                continue;
             }
         }
-        ++in;
+        return true;
     }
-    if (in != out)
-        str->truncate(out - str->utf16());
+    return false;
 }
 
 static bool isBidirectionalRorAL(uint uc)
@@ -2084,7 +2084,10 @@ Q_AUTOTEST_EXPORT void qt_nameprep(QString *source, int from)
                         firstNonAscii > from ? firstNonAscii - 1 : from);
 
     // Strip prohibited output
-    stripProhibitedOutput(source, firstNonAscii);
+    if (containsProhibitedOuptut(source, firstNonAscii)) {
+        source->resize(from);
+        return;
+    }
 
     // Check for valid bidirectional characters
     bool containsLCat = false;
diff --git a/tests/auto/corelib/io/qurlinternal/tst_qurlinternal.cpp b/tests/auto/corelib/io/qurlinternal/tst_qurlinternal.cpp
index fa3dec620a..d60e1b6647 100644
--- a/tests/auto/corelib/io/qurlinternal/tst_qurlinternal.cpp
+++ b/tests/auto/corelib/io/qurlinternal/tst_qurlinternal.cpp
@@ -351,7 +351,7 @@ void tst_QUrlInternal::nameprep_testsuite_data()
         << QString() << 0 << 0;
 
     QTest::newRow("Non-ASCII multibyte space character U+1680")
-        << QString::fromUtf8("\xE1\x9A\x80")
+        << QString::fromUtf8("x\xE1\x9A\x80x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
@@ -376,12 +376,12 @@ void tst_QUrlInternal::nameprep_testsuite_data()
         << QString() << 0 << 0;
 
     QTest::newRow("Non-ASCII 8bit control character U+0085")
-        << QString::fromUtf8("\xC2\x85")
+        << QString::fromUtf8("x\xC2\x85x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Non-ASCII multibyte control character U+180E")
-        << QString::fromUtf8("\xE1\xA0\x8E")
+        << QString::fromUtf8("x\xE1\xA0\x8Ex")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
@@ -391,47 +391,47 @@ void tst_QUrlInternal::nameprep_testsuite_data()
         << QString() << 0 << 0;
 
     QTest::newRow("Non-ASCII control character U+1D175")
-        << QString::fromUtf8("\xF0\x9D\x85\xB5")
+        << QString::fromUtf8("x\xF0\x9D\x85\xB5x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Plane 0 private use character U+F123")
-        << QString::fromUtf8("\xEF\x84\xA3")
+        << QString::fromUtf8("x\xEF\x84\xA3x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Plane 15 private use character U+F1234")
-        << QString::fromUtf8("\xF3\xB1\x88\xB4")
+        << QString::fromUtf8("x\xF3\xB1\x88\xB4x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Plane 16 private use character U+10F234")
-        << QString::fromUtf8("\xF4\x8F\x88\xB4")
+        << QString::fromUtf8("x\xF4\x8F\x88\xB4x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Non-character code point U+8FFFE")
-        << QString::fromUtf8("\xF2\x8F\xBF\xBE")
+        << QString::fromUtf8("x\xF2\x8F\xBF\xBEx")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Non-character code point U+10FFFF")
-        << QString::fromUtf8("\xF4\x8F\xBF\xBF")
+        << QString::fromUtf8("x\xF4\x8F\xBF\xBFx")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Surrogate code U+DF42")
-        << QString::fromUtf8("\xED\xBD\x82")
+        << QString::fromUtf8("x\xED\xBD\x82x")
         << QString()
         << QString("Nameprep") << 0 <<  STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Non-plain text character U+FFFD")
-        << QString::fromUtf8("\xEF\xBF\xBD")
+        << QString::fromUtf8("x\xEF\xBF\xBDx")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Ideographic description character U+2FF5")
-        << QString::fromUtf8("\xE2\xBF\xB5")
+        << QString::fromUtf8("x\xE2\xBF\xB5x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
@@ -441,22 +441,22 @@ void tst_QUrlInternal::nameprep_testsuite_data()
         << QString() << 0 << 0;
 
     QTest::newRow("Left-to-right mark U+200E")
-        << QString::fromUtf8("\xE2\x80\x8E")
-        << QString::fromUtf8("\xCC\x81")
+        << QString::fromUtf8("x\xE2\x80\x8Ex")
+        << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Deprecated U+202A")
-        << QString::fromUtf8("\xE2\x80\xAA")
-        << QString::fromUtf8("\xCC\x81")
+        << QString::fromUtf8("x\xE2\x80\xAA")
+        << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Language tagging character U+E0001")
-        << QString::fromUtf8("\xF3\xA0\x80\x81")
-        << QString::fromUtf8("\xCC\x81")
+        << QString::fromUtf8("x\xF3\xA0\x80\x81x")
+        << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
     QTest::newRow("Language tagging character U+E0042")
-        << QString::fromUtf8("\xF3\xA0\x81\x82")
+        << QString::fromUtf8("x\xF3\xA0\x81\x82x")
         << QString()
         << QString("Nameprep") << 0 << STRINGPREP_CONTAINS_PROHIBITED;
 
@@ -512,12 +512,6 @@ void tst_QUrlInternal::nameprep_testsuite()
     QFETCH(QString, out);
     QFETCH(QString, profile);
 
-    QEXPECT_FAIL("Left-to-right mark U+200E",
-                 "Investigate further", Continue);
-    QEXPECT_FAIL("Deprecated U+202A",
-                 "Investigate further", Continue);
-    QEXPECT_FAIL("Language tagging character U+E0001",
-                 "Investigate further", Continue);
     qt_nameprep(&in, 0);
     QCOMPARE(in, out);
 }
@@ -549,9 +543,9 @@ void tst_QUrlInternal::nameprep_highcodes_data()
             << QString() << 0 << 0;
     }
     {
-        QChar st[] = { 'D', 0xdb40, 0xdc20, 'o', 0xd834, 0xdd7a, '\'', 0x2060, 'h' };
+        QChar st[] = { 'D', 'o', '\'', 0x2060, 'h' };
         QChar se[] = { 'd', 'o', '\'', 'h' };
-        QTest::newRow("highcodes (D, U+E0020, o, U+1D17A, ', U+2060, h)")
+        QTest::newRow("highcodes (D, o, ', U+2060, h)")
             << QString(st, sizeof(st)/sizeof(st[0]))
             << QString(se, sizeof(se)/sizeof(se[0]))
             << QString() << 0 << 0;